[From  The  American  Journal  of  Science,  Vol.  XXIX,  June,  1910.] 


CONTRIBUTIONS  to  the  GEOLOGY  of  the 
GRAND  CANYON,  ARIZONA.— The 
GEOLOGY  of  the  SHINUMO 

AREA. 


% 


S'  A  ^ 

By  L.  F.  Noble. 
(With  Plate  VJ 


(Contributions  from  the  Geological  Department, 
Yale  University,  New  Haven,  Conn*,  U*  S.  A*) 


b  b  I 

N  &<o  c. 


THE 

AMERICAN  JOURNAL  OF  SCIENCE 

[FOURTH  SERIES.] 


Art.  XXXII. — Contributions  to  the  Geology  of  the  Grand 
Canyon ,  Arizona. — The  Geology  of  the  Shinumo  Area  • 
by  L.  F.  Noble. 


Part  I. 

Preface. 

Introduction. 

Location. 

Field  Work. 

Literature. 

Acknowledgments. 

Geologic  Nomenclature. 

Topography,  Climate  and  Vegetation. 
General  Geology. 


Archean —  Vishnu  Schist. 

Name. 

Distribution  in  the  Kaibab  Division. 

Occurrence  and  Distribution  in  the  Shinumo  Area. 
Lithology. 

Origin. 

Age  and  Correlation. 

Bibliography. 


Preface. 

Exposures  of  unaltered  pre-Cambrian  sediments  are  of  great 
interest  to  the  geologist  in  any  part  of  the  world.  Especially 
is  this  true  in  the  Grand  Canyon  of  Arizona,  where  the  cer¬ 
tainty  of  stratigraphic  position  and  completeness  of  exposure 
are  almost  without  a  parallel,  and  where  the  spectacular  nature 
of  the  occurrence  partakes  of  the  stupendous  and  fantastic 
scenery  of  that  mighty  gorge,  from  the  spell  of  which  the 
observer  is  never  entirely  free.  During  a  pleasure  trip  in  the 
Grand  Canyon  region  in  January,  1908,  in  company  with 
Professor  H.  E.  Gregory  of  Yale  and  others,  the  writer  chanced 
to  visit  a  hitherto  undescribed  exposure  of  these  rocks  on  the 

Am.  Jour.  Sci. — Fourth  Series,  Vol.  XXIX,  No.  173. — May,  1910. 

25 


370  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

Colorado  River  at  the  foot  of  Bass  Trail.  Although  only  a 
few  hours  were  spent  in  the  vicinity,  the  diagrammatic  sim¬ 
plicity  with  which  the  structural  relations  were  revealed  was 
such  that  the  interest  could  be  immediately  realized.  In  the 
summer  of  that  year  the  writer  returned  with  the  intention 
of  making  a  detailed  study  of  these  pre-Cambrian  rocks.  He 
speedily  found  that  the  interest  in  this  part  of  the  Grand 
Canyon  did  not  end  with  the  study  of  the  pre-Cambrian  alone : 
it  was  found  that  the  line  of  displacement  of  the  West  Kaibab 
fault  which  runs  into  the  Grand  Canyon  at  this  point  was  itself  a 
remarkable  structural  feature  and  worthy  of  considerable  atten¬ 
tion  ;  that  the  entire  Paleozoic  section  of  the  canyon  wall  was 
here,  as  elsewhere,  undescribed  in  absolute  detail ;  and  finally 
that  there  were  problems  of  physiographic  interest  in  the  Grand 
Canyon  region  the  ultimate  solution  of  which  could  be  greatly 
helped  by  a  detailed  study  at  this  point.  It  was  therefore  de¬ 
cided  to  extend  the  work  to  a  complete  areal  study  of  the 
region,  including  for  this  purpose  the  greater  part  of  the  new 
Sliinumo  topographic  sheet  of  the  United  States  Geological 
Survey.  This  report,  accompanied  by  an  atlas  containing  the 
geologic  maps,  sections,  and  plates,  was  presented  to  the  faculty 
of  the  Graduate  Department  of  Yale  University  in  June,  1909, 
as  a  thesis  in  partial  fulfillment  of  the  requirements  for  the 
degree  of  Ph.D.,  under  the  title  of  “  The  Geology  of  the 
Sliinumo  Area,  Grand  Canyon,  Arizona.” 

It  is  intended  to  present  the  substance  of  the  thesis  in  two 
or  more  articles  in  this  Journal.  The  entire  report  is  to  be 
published  in  extenso  at  a  later  date  as  a  Bulletin  of  the  United 
States  Geological  Survey.  For  this  reason,  as  well  as  because 
of  the.  large  scale  and  elaborate  nature,  the  geologic  map,  sec¬ 
tions,  and  plates  will  not  be  published  here. 

The  first  article  will  be  presented  in  two  parts.  Part  I,  now 
published,  comprises  a  general  introductory  description  of  the 
Sliinumo  area  and  deals  with  the  basement  rocks  of  the  Archean. 
Part  II,  to  follow  in  a  later  number,  will  be  devoted  to  the 
Algonkian  rocks  of  the  Grand  Canyon  series  and  will  include 
a  map  showing  the  distribution  of  the  Vishnu  and  Grand 
Canyon  series  in  the  Grand  Canyon. 

It  is  also  proposed  to  devote  a  later  article  or  articles  to  the 
Paleozoic  section  of  the  canyon  wall ;  to  a  structural  study  of 
the  area,  including  both  the  pre-Cambrian  and  later  structure 
and  a  study  of  the  displacements  on  the  line  of  the  West 
Kaibab  fault ;  and  finally  to  features  of  physiographic  interest 
in  the  area, — such  as  the  drainage  system  of  the  plateaus  and 
the  origin  of  the  Esplanade  and  Tonto  platforms. 


The  Shinurno  Area. 


371 


Introduction. 

Location  and  geography. — The  area  studied  lies  in  Coconino 
County  in  the  northern  part  of  Arizona  in  what  is  known  as 
the  Plateau  Province  of  the  Territory.  It  is  inclosed  between 
meridians  112°  25r  and  112°  15r  and  parallels  36°  20'  and  36° 
05',  and  includes  about  150  square  miles.  It  comprises  rather 
more  than  the  eastern  half  of  the  area  of  the  Shinurno  topo¬ 
graphic  sheet  of  the  United  States  Geological  Survey. 

The  greater  part  of  the  area  lies  within  the  depths  of  the 
Grand  Canyon  and  includes  the  Shinurno  Amphitheater,  which 
is  the  largest  of  the  great  amphitheaters  in  the  Kaibab  divi¬ 
sion.  The  eastern  and  northern  parts  of  the  area  include  that 
part  of  the  “  mainland  ”  north  of  the  canyon  which  forms  the 
immediate  rim  of  the  Shinurno  Amphitheater,  reaching  from 
the  promotory  of  Point  Sublime  on  the  east  to  the  Muav 
Saddle  on  the  west;  this  northern  mainland  is  a  part  of  the 
Kaibab  Plateau.  The  northwestern  part  of  the  area  includes 
the  surface  of  Powell  Plateau,  a  great  butte,  or  u  island,”  lying 
athwart  the  course  of  the  Grand  Canyon.  The  southwest  cor¬ 
ner  includes  a  part  of  the  southern  “  mainland  ”  which  cul¬ 
minates  in  the  great  promontory  known  as  Havasupai  Point ; 
this  southern  mainland  belongs  to  the  Coconino  Plateau. 

About  a  mile  west  of  Havasupai  Point  is  situated  Bass  Camp, 
the  only  habitation  in  the  area.  The  camp  was  established  by 
Mr.  W.  W.  Bass  some  twenty -five  years  ago  to  accommodate 
the  tourist  traffic.  From  this  point  a  trail  was  constructed  to 
the  river,  which  was  crossed  by  boat  at  times  of  low  water. 
A  permanent  camp  was  established  in  the  bottom  of  the  can¬ 
yon  on  the  north  side  of  the  river  about  a  mile  up  Shinurno 
Creek,  where  an  irrigated  garden  was  made  upon  the  site  of  an 
ancient  one  cultivated  by  the  prehistoric  inhabitants  of  the 
region.  Later  a  trail  was  constructed  up  Muav  Canyon  to  the 
north  rim,  by  means  of  which  access  may  be  had  to  points  on 
the  Kaibab  Plateau  and  the  settlements  in  Southern  Utah. 
Recently  deposits  of  copper  and  asbestos  have  been  discovered 
by  Mr.  Bass  in  the  depths  of  the  canyon,  to  which  he  has  con¬ 
structed  additional  trails.  In  March,  1908,  three  wire  cables 
were  suspended  across  the  granite  gorge  at  the  foot  of  Bass 
Trail.  On  these  cables  travels  a  wire  car,  so  that  at  the  present 
time  the  Colorado  River  may  be  crossed  at  this  point  regard¬ 
less  of  high  water.  Bass  Camp  is  most  easily  reached  by 
wagon  road  from  Bass  Station  on  the  Grand  Canyon  branch  of 
the  Sante  Fe  Railroad,  a  distance  of  20  miles  southeast ;  or 
from  the  El  Tovar  Hotel  at  the  terminus  of  the  railroad  25 
miles  east.  Another  road  leads  20  miles  southwest  to  the  rim 
of  Cataract  Canyon,  into  which  a  trail  leads  down  to  the  Supai 


372  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

Indian  village.  Roads  also  lead  to  the  towns  of  Williams  and 
Ash  Fork,  about  60  miles  respectively  south  and  southwest. 

Field  Work. — The  geological  field  work  upon  which  the 
thesis  is  based  was  begun  upon  the  23d  of  August,  1908,  and 
ended  upon  the  12th  day  of  December  of  the  same  year.  The 
greater  part  of  the  work  was  done  from  a  permanent  camp  on 
the  Shinumo,  which  served  as  a  base  of  operations  for  studying 
all  the  region  in  the  greater  depths  of  the  canyon.  Two  trips 
were  made  to  the  north  rim.  The  first  was  made  in  September 
and  the  second  in  November.  Each  trip  lasted  a  week.  Camp 
was  established  in  the  Muav  Saddle  and  excursions  were  made 
from  there  over  the  surface  of  Powell  Plateau  and  over  the 
Kaibab  mainland  as  far  as  the  head  of  the  Shinumo  Amphi¬ 
theater.  The  writer  was  accompanied  on  these  occasions,  as 
well  as  during  his  stay  in  camp  on  the  Shinumo,  by  Mr.  John 
Walthenberg  as  guide,  whose  detailed  knowledge  of  the  region 
and  whose  assistance  in  other  ways  were  invaluable.  The  work 
in  the  southern  part  of  the  area  was  done  from  Bass  Camp, 
from  which  point  excursions  were  made  over  the  Coconino 
Plateau  and  into  the  upper  part  of  the  canyon. 

Literature. — Two  references  to  the  geology  of  the  area  are 
to  be  found  in  geological  literature : 

Captain  Dutton  in  his  monograph  entitled  “The  Tertiary 
History  of  the  Grand  Canyon  District”  (Dutton,  a)  describes 
most  fully  and  charmingly  the  geology  of  the  north  rim  in 
this  section  of  the  Grand  Canyon.  In  Chapter  YII  he  describes 
the  surface  features  and  scenery  of  the  Ivaibab  Plateau  in  the 
vicinity  of  Point  Sublime.  Chapter  VIII  is  devoted  to  the 
panorama  disclosed  from  Point  Sublime,  while  Chapter  IX 
describes  the  walls  of  the  amphitheaters  of  the  north  side  in 
detail.  The  Muav  Saddle  and  Powell  Plateau  are  described 
on  pp.  162-167,  and  the  Shinumo  Amphitheater  on  pp.  167-174. 
His  work,  however,  did  not  extend  into  the  depths  of  the 
canyon. 

Mr.  J.  S.  Diller  of  the  United  States  Geological  Survey 
describes  in  his  report  on  the  production  of  asbestos  in  u  Min¬ 
eral  Resources  for  1907”  (Diller,  a)  the  deposits  of  asbestos 
occurring  in  the  Algonkian  sediments  of  this  area  near  Bass 
Ferry.  This  is  the  only  reference  in  the  literature  to  the 
presence  of  Algonkian  strata  in  this  part  of  the  Grand  Canyon. 

Acknowledgments. — In  1901  Mr.  Charles  D.  Walcott  and 
Mr.  G.  K.  Gilbert  spent  several  days  at  Mr.  Bass’s  camp  on  the 
81iinumo  and  at  that  time  worked  out  the  structure  of  the 
pre-Cambrian  sediments,  which  Mr.  Walcott  correlated  with 
the  section  described  by  him  in  Unkar  Valley  (Walcott,  a,  b , 
c,  d ,  and  e).  His  notes,  however,  are  unpublished,  and  it  is 
due  to  his  kindness  and  courtesy  that  the  writer  is  enabled  to 
present  the  first  description  of  the  area.  To  Mr.  Walcott  the 


The  Shinumo  Area. 


373 


writer  is  also  indebted  for  the  identification  of  Cambrian 
fossils,  for  a  list  of  the  Cambrian  fossils  found  in  the  region, 
and  for  assistance  in  interpreting  the  stratigraphy. 

To  Professor  Joseph  Barrell,  to  Professor  Charles  Schu- 
chert,  and  to  Professor  Louis  V.  Pirsson,  all  of  Yale  University, 
the  most  sincere  thanks  are  due  for  continued  interest  and 
advice  during  all  stages  of  the  work.  To  Mr.  W.  W.  Bass  the 
writer  is  indebted  for  material  assistance  and  guidance  during 
his  work  in  the  field. 

Geologic  Nomenclature. — The  geologic  names  employed  in 
this  article  are  revised  names  recently  authorized  by  the 
United  States  Geological  Survey,  with  the  exception  of  those  of 
the  Cambrian,  which  as  yet  have  not  been  formally  revised. 
The  first  table  below  gives  the  rock  formations  of  the  Shinumo 
area  in  terms  of  the  revised  terminology,  while  the  second 
table  gives  the  equivalent  of  the  newer  in  the  older  nomen¬ 
clature. 


System 


I. 

Geologic  column  of  the  Shinumo  Area. 

Series  Group  Formation 

r 


Carboniferous 


f  Kaibab  limestone 

Pennsylvanian  ...  Aubrey  . .  Coconino  sandstone 

(_  Supai  formation 

Pennsylvanian  and  Redwa]1  __  Redwall  liraestone 
Mississippian 

(Unconformity  of  erosion  without  uniformity  of  dip) 

“  Marbled  limestone” 


Cambrian  . 


Acadian 

(and  Saratogan?) 


.  Tonto  _ . 


“  Tonto  shale  ” 

“  Tonto  sandstone  ” 


(Great  angular  unconformity) 

Algonkian _ Grand  Canyon  ..T  Unkar 

(Greatest  angular  unconformity) 

Archean . .  Vishnu 


IT. 

Classic  Nomenclature  of  the  reports  of 
Revised  Nomenclature  Dutton ,  Gilbert ,  and  Walcott. 


Formation  '  Formation  Group 

Kaibab  limestone _ Cherty  limestone - }  tt  Anhrev 

Coconino  sandstone _ Cross-bedded  sandstone  ..  \  '  *  ^ 

Supai  formation.  . Lower  Aubrey  sandstone. _  Lower  Aubrey 

Redwall  limestone. _ Redwall  limestone — . —  Redwall 

“Marbled  limestone”  ..  Marbled  limestone  - - |  tT  T  t 

•‘Tonto  shale” . .  Tonto  shale  . j  Uppel  i0  10 

a  Tonto  sandstone  ” _ Tonto  sandstone . —  Lower  Tonto 


374  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

Topography ,  Climate ,  Vegetation. 

The  surfaces  of  the  plateaus  in  the  Shinumo  area  through 
which  the  pathway  of  the  Grand  Canyon  is  trenched  are  every¬ 
where  developed  at  the  same  horizon  on  the  highest  member 
of  the  Paleozoic  series  occurring  in  the  Canyon  wall  and  known 
as  the  Kaibab  limestone,  the  Mesozoic  and  Tertiary  formations 
having  been  stripped  back  to  the  terraces  of  southern  Utah. 

The  surface  of  the  southern,  or  Coconino,  plateau  slopes  to 
the  southwest  away  from  the  canyon  rim  at  the  rate  of  about 
200  feet  to  the  mile.  The  drainage  system  of  the  plateau 
surface  consists  of  a  series  of  mature,  open-floored  valleys  with 
gently  sloping  sides,  which  contain  no  living  streams.  These 
valleys  trend  southwesterly  away  from  the  canyon  rim  with 
the  slope  of  the  plateau  surface  and  drain  into  the  broad,  shal¬ 
low  synclinal  basin  occupied  by  Cataract  Canyon.  In  tracing 
one  of  these  mature  valleys  toward  its  head,  it  is  a  common 
thing  to  find,  it  truncated  as  a  hanging  valley  by  the  wall  of 
the  Grand  Canyon.  So  general  is  this  phenomenon,  that  the 
stranger  who  loses  his  way  on  the  southern  plateau  has  only  to 
keep  in  mind  that  if  he  follow  any  valley  far  enough  headward 
he  will  come  out  upon  the  rim  of  the  Grand  Canyon. 

The  surface  of  the  northern,  or  Kaibab,  plateau  is  in  every 
respect  similar  to  that  of  the  Coconino.  The  same  system  of 
mature  valleys  covers  its  surface,  which  slopes  southwesterly  to 
the  rim  of  the  canyon.  The  only  difference  is  that  the  surface 
drainage  runs  into  the  Grand  Canyon  instead  of  away  from  it. 
Neither  plateau  surface  contains  a  living  stream. 

Powell  Plateau  may  be  regarded  as  a  dismembered  part  of 
the  Kaibab.  Its  surface  is  developed  at  the  same  horizon  on 
the  Kaibab  limestone,  slopes  southwesterly  at  the  rate  of  900 
feet  in  five  miles,  and  contains  the  same  mature  drainage 
system.  It  is  substantially  a  great  island,  surrounded  on  three 
sides  by  mile-deep  canyons,  and  isolated  from  the  mainland  by 
erosion  in  the  line  of  displacement  of  the  West  Kaibab  fault. 
The  gap  that  separates  Powell  Plateau  from  the  mainland  is 
calied  the  Muav  Saddle,  and  is  notched  800  feet  below  the 
surface  of  the  plateaus.  No  more  striking  topographic  con¬ 
trast  can  be  imagined  than  that  between  the  mature  drainage 
systems  of  the  plateaus  and  the  youthful  topography  within 
the  deep- trenched  canyon. 

The  strata  of  the  Paleozoic  rock  system  within  the  area  dip 
gently  and  almost  uniformly  to  the  southwest  at  the  rate  of 
about  200  feet  to  the  mile,  and  the  surfaces  of  the  plateaus  are 
everywhere  accordant  with  the  rock  structure.  A  slight  local 
interruption  occurs  where  the  line  of  the  West  Kaibab  fault 
crosses  the  area,  along  which  the  strata  are  flexed  into  a  mono- 


The  Shinumo  Area. 


375 


cline  clipping  sliarply  northeastward.  A  few  minor  flexures 
cross  the  area  in  a  northwesterly  direction ;  all  dip  to  the 
southwest  and  are  hardly  more  than  gentle  swells  upon  the 
general  warping. 

The  Shinumo  Area  is  a  critical  area  for  the  study  of  the 
topography  within  the  canyon  itself.  It  is  here  that  the  topog¬ 
raphy  undergoes  a  transition  from  the  profile  which  is  char¬ 
acteristic  of  the  Ivaibab  division  to  that  which  is  characteristic 
of  the  Kanab. 

In  the  eastern,  or  Ivaibab,  division  of  the  canyon,  the  de¬ 
scent  of  the  wall  is  unusually  abrupt  throughout  the  entire 
Paleozoic  rock  series.  The  only  bench  or  terrace  of  any 
extent  or  definition  is  that  which  is  developed  near  the  bottom 
of  the  canyon  upon  the  summit  of  the  basal  sandstone  of  the 
Tonto  group.  The  bench  is  known  as  the  Tonto  platform  ;  in 
it  is  cut  an  inner  gorge,  in  which  the  river  flows  upon  the  base¬ 
ment  schists  of  the  Archean.  The  platform  averages  a  mile 
in  width  on  either  side  of  the  canyon,  and  is  so  well  defined 
that  one  may  travel  upon  it  throughout  the  length  of  the 
Ivaibab  division.  In  the  next  western,  or  Kanab,  division  of 
the  canyon  the  lower  terrace  has  disappeared.  Instead,  a 
wide  bench  is  opened  out  upon  the  summit  of  the  hard  red 
sandstones  of  the  Supai  formation  of  the  Aubrey  group,  about 
a  thousand  feet  below  the  level  of  the  canyon  rim.  This 
bench  forms  a  broad  level  platform  averaging  two  miles  in 
width  on  either  side  of  the  canyon,  and  has  been  named  by 
Dutton  the  ‘‘Esplanade.”  Through  it  is  cut  a  deep  and  narrow 
inner  gorge,  at  the  bottom  of  which  flows  the  river. 

The  topography  of  the  Ivaibab  division  is  characterized  by 
a  much  greater  dissection  than  that  of  the  Ivanab ;  great 
amphitheaters  are  eroded  back  into  the  north  wall,  thronged 
with  buttes  and  outliers  fashioned  out  of  every  formation  of  the 
Paleozoic  rock  series  and  trenched  by  a  multitude  of  side 
gorges.  The  topography  of  the  Kanab  division  is  very  simple, 
consisting  only  of  a  broad  outer  canyon  in  which  is  cut  the 
inner  gorge;  the  great  dissection  which  makes  the  fantastic 
scenery  of  the  Ivaibab  division  is  entirely  lacking. 

Eastward  from  Havasupai  Point  in  the  Shinumo  area  the 
topography  in  the  canyon  is  that  which  is  characteristic  of  the 
Ivaibab  division.  Westward,  however,  the  upper  platform  or 
“Esplanade”  begins  to  appear.  The  Shinumo  amphitheater, 
which  occupies  the  greater  part  of  the  area  to  be  described, 
presents  a  combination  of  both  types  of  topography  ;  the  great 
dissection  and  the  presence  of  the  Tonto  platform  are  features 
characteristic  of  the  Ivaibab  division,  but  the  Esplanade  charac¬ 
teristic  of  the  Kanab  division  is  developed  to  quite  an  equal 
extent  upon  the  upper  surface  of  the  Supai  sandstone.  In  the 


376  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

western  part  of  the  area  the  Tonto  platform  fades  out,  the 
river  closes  in  a  deep  inner  gorge  in  the  Esplanade,  and  the 
profile  of  the  canyon  becomes  that  of  the  Kanab  division, — 
the  view  of  the  canyon  westward  from  Bass  Camp  discloses  a 
broad  expanse  of  Esplanade  for  fifteen  miles,  through  which 
the  inner  gorge  sweeps  in  two  great  meanders  almost  from  wall 
to  wall.  In  the  center  of  the  Shinumo  area  the  Esplanade  is 
rather  more  dissected  than  in  the  Kanab  division  farther  west, 
but  its  identity  as  a  topographic  feature  is  already  well 
established. 

The  floors  of  the  Esplanade  and  Tonto  platforms,  like  the 
surfaces  of  the  plateaus,  are  structural  surfaces  developed 
everywhere  at  the  same  horizon  on  the  underlying  formation. 
In  conformity  with  the  dip  of  the  rock  system  they  likewise 
slope  to  the  southwest  at  the  rate  of  about  200  feet  to  the 
mile. 

Another  topographic  contrast  is  presented  in  the  difference 
in  dissection  of  the  two  sides  of  the  Grand  Canyon,  a  feature 
which  is  characteristic  of  the  entire  pathway  through  the 
Kaibab.  The  north  rim  lies  three  times  as  far  back  from  the 
river  as  the  south  rim  ;  the  great  amphitheaters  with  their 
limiting  promonotories  extending  far  into  the  canyon,  the 
buttes  and  temples,  and  the  deep  lateral  gorges  all  belong  to 
the  north  side  of  the  canyon.  The  south  wall  presents  a 
simple  aspect :  the  side  gorges  rarely  extend  back  into  the  rim 
of  the  canyon,  there  are  few  buttes  and  outliers,  and  the  great 
amphitheaters  are  wholly  lacking.  Compared  with  the  fan¬ 
tastic  topography  of  the  north  side,  the  scenic  effect  of  the 
south  wall  is  precipitous  and  somber. 

The  Colorado  Biver  enters  the  area  in  the  southeast  corner, 
flows  northwestwardly  to  the  center  of  the  area,  and  turns 
sharply  west  at  the  point  where  it  is  joined  by  the  Shinumo. 
The  Shinumo  is  the  only  living  tributary  in  the  area,  and  is 
the  master  stream  that  drains  the  Shinumo  amphitheater  of  the 
north  wall.  It  is  a  stream  of  clear  water  of  the  same  order  of 
magnitude  as  Bright  Angel  Creek  twenty  miles  east,  and  is  in 
striking  contrast  to  the  muddy  Colorado. 

The  topography  of  the  Shinumo  amphitheater,  besides  pre¬ 
senting  an  equal  development  of  both  the  Esplanade  and  Tonto 
platforms,  is  remarkable  in  another  way.  In  the  other  great 
amphitheaters  of  the  Kaibab  division  the  master  gorges  trend 
to  the  southwest ;  the  tributary  gorges  trend  in  the  same  general 
direction  and  lateral  gorges  perpendicular  to  the  main  axes  of 
the  amphitheaters  are  of  minor  development.  In  the  Shinumo 
amphitheater  the  lateral  gorges  have  become  the  dominant 
feature,  so  that  the  main  axis  of  the  amphitheater  trends  to  the 
northwest  at  right  angles  to  the  course  of  the  master  stream 


The  Shinumo  Area. 


377 


and  parallel  to  the  course  of  the  Colorado  River.  The  greatest 
lateral  gorge  extends  entirely  across  the  Shinumo  amphitheater 
from  Point  Sublime  on  the  southeast  to  the  Muav  Saddle  on 
the  northwest.  Only  a  mile  of  this  lateral  gorge  is  occupied 
by  the  master  stream  of  the  Shinumo ;  the  remainder  is  occupied 
by  two  small  intermittent  streams.  The  western  part  of  the 
gorge  which  extends  from  the  Muav  Saddle  to  Shinumo  Creek 
is  drained  by  Muav  Creek  and  is  called  the  “  Muav  Canyon  ”  ; 
the  eastern  part  extending  from  the  Shinumo  to  Point  Sublime 
is  drained  by  Flint  Creek  and  has  no  local  name.  The  entire 
lateral  gorge  will  be  referred  to  as  the  “  Muav-Flint  Creek 
Canyon.”  This  remarkable  linear  depression  is  situated  upon 
the  line  of  the  West  Kaibab  fault  and  has  been  conditioned  bv 
erosion  in  that  line  of  fracture.  Two  smaller  lateral  gorges 
cross  the  heart  of  the  amphitheater  parallel  to  the  “  Muav- 
Flint  Creek  Canyon  ” ;  these  are  situated  upon  minor  lines  of 
fracture. 

It  may  be  noted  here  that  Tapeats  amphitheater  to  the  west¬ 
ward,  in  the  Kanab  division  of  the  canyon,  is  characterized  by 
similar  topographic  features  where  it  is  'crossed  by  the  West 
Kaibab  fault. 

In  general  the  surface  of  the  Kaibab  plateau  at  the  rim  of 
the  canyon  is  about  1000  feet  higher  than  that  of  the  Coconino 
plateau  directly  opposite.  But  since  the  surfaces  of  the  plateaus 
and  of  the  platforms  within  the  canyon  are  structural  surfaces, 
elevations  diminish  progressively  southwestward  in  all  parts  of 
the  area  in  accordance  with  the  dip  of  the  underlying  rock 
structure.  For  example  :  the  elevation  of  the  Kaibab  Plateau 
at  the  head  of  the  Shinumo  amphitheater  is  8000  feet ;  at  the 
end  of  the  promontory  of  Point  Sublime,  four  miles  southward, 
it  is  7500  feet.  The  elevation  of  the  Coconino  Plateau  at  the 
end  of  Havasupai  Point  is  6750  feet;  at  Bass  Camp,  a  mile 
southwestward,  6652  feet.  The  eastern  end  of  Powell  Plateau 
at  Dutton  Point  is  7555  feet;  the  western  end  is  6600  feet. 
Within  the  canyon  the  elevation  of  the  Esplanade  platform  on 
the  north  side  of  the  river  below  Point  Sublime  is  6250  feet ; 
on  the  south  side  of  the  river  below  Bass  Camp,  5400  feet.  » 
In  the  eastern  part  of  the  area  the  Ton  to  platform  is  3500  feet ; 
in  the  western  part,  2700  feet. 

The  elevation  of  the  surface  of  the  Colorado  River  where  it 
enters  the  area  on  the  east  is  2250  feet ;  where  it  leaves  the 
area  on  the  west,  2150  feet, — a  drop  of  100  feet  in  10  miles. 
The  river  at  Bass  Ferry  is  300  feet  wide  and  50  feet  deep,  with 
a  rise  of  40  feet  at  time  of  high  water. 

The  greatest  drops  in  the  shortest  linear  distance  in  the  area 
are  at  Dutton  Point,  where  a  drop  of  5355  feet  to  the  river  is 
accomplished  in  a  distance  of  three  miles ;  and  at  Havasupai 


378  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

Point,  where  a  drop  of  4500  feet  is  accomplished  in  a  distance 
of  a  mile  and  a  half.  The  latter  drop  is  not  equaled  elsewdiere 
in  the  Grand  Canyon. 

The  width  of  the  Grand  Canyon  in  the  Shinumo  area  from 
Bass  Camp  to  Dutton  Point  is  seven  miles ;  from  Bass  Camp 
to  the  head  of  the  Shinumo  Amphitheater  it  is  twelve  miles ; 
from  Havasupai  Point  to  Point  Sublime  it  is  five  and  one-half 
miles.  The  latter  is  the  minimum  width  of  the  Grand  Canyon 
in  the  Kaibab  division.  Even  at  this,  the  narrowest  point,  the 
width  is  over  five  times  the  depth. 

The  differences  of  climate  to  be  found  within  the  Shinumo 
area  are  remarkable.  The  range  in  climate  between  the  Kai¬ 
bab  Plateau  on  the  north  rim  and  the  bottom  of  the  canyon  is 
as  great  as  that  between  the  mountains  of  Colorado  and  the 
Mojave  Desert.  The  winters  on  the  Kaibab  are  extremely 
severe  :  from  November  to  April  the  snow  lies  deep  in  the 
woods,  often  accumulating  to  a  depth  of  ten  feet ;  even  in  mid¬ 
summer  the  nights  are  chilly  and  days  delightfully  cool.  Within 
the  canyon,  however,  the  snow  rarely  falls  below  the  level  of 
the  Esplanade,  while  on  the  Tonto  platform  a  fall  of  snow  is 
practically  unknown.  The  winters  in  the  depths  of  the  can¬ 
yon  are  mild,  and  freezing  temperatures  are  rare.  From  April 
to  October  the  entire  canyon  below  the  Bed  Wall  concentrates 
the  solar  heat,  transforming  it  into  a  veritable  oven ;  all  day 
the  bare  rocks  absorb  the  heat  of  the  sun,  becoming  so  hot  as 
to  burn  the  hand  ;  by  nightfall  the  wind  becomes  a  furnace 
blast,  and  until  after  midnight  the  rocks  radiate  their  heat, 
feeding  the  hot  wind,  which  blows  without  ceasing.  The 
effect  of  the  heat,  however,  is  not  enervating,  for  the  dryness 
of  the  warm  wind  evaporates  all  moisture  from  the  body  and 
keeps  it  cool.  The  climate  of  the  southern,  or  Coconino,  Pla¬ 
teau  at  Bass  Camp  is  characterized  by  more  open  winters  than 
the  Kaibab,  as  well  as  by  warmer  summers  :  the  snow  in  winter 
rarely  accumulates  on  the  surface  to  a  great  depth,  and  as  a 
rule  vanishes  entirely  within  three  days  after  a  storm  ;  in  sum¬ 
mer  the  days  are  unpleasantly  hot. 

The  climate  of  the  Kaibab  Plateau  is  decidedly  moist,  the 
precipitation  being  probably  twice  as  great  as  that  received 
upon  the  Coconino  Plateau  across  the  canyon  ;  this  is  chiefly 
due  to  its  greater  altitude.  In  winter  the  precipitation  takes 
the  form  of  snow  ;  in  summer  it  comes  in  the  form  of  showers, 
which  occur  through  the  afternoon  and  evening.  Looking 
across  the  canyon  from  Bass  Camp  on  the  south  rim,  almost 
any  summer  evening  one  may  see  storm  after  storm  sweeping 
over  the  Kaibab  surface,  usually  accompanied  by  violent  elec¬ 
trical  display,  while  the  sky  overhead  and  to  the  westward 
over  the  Kanab  Desert  remains  as  clear  as  crystal.  Another 


The  Shinumo  Area. 


379 


phenomenon  that  contributes  to  the  greater  rainfall  of  the 
north  rim  seems  to  be  the  presence  of  the  Grand  Canyon  itself : 
after  every  general  storm  that  visits  both  sides  of  the  canyon 
alike,  there  follows  on  the  south  rim  a  day  of  clearing;  but  the 
clouds  that  rise  out  of  the  canyon  after  the  storm  sweep  back 
over  the  north  wall  and  re-precipitate  on  the  Kaibab  surface ; 
these  secondary  storms  almost  never  return  over  the  south  rim. 
The  climate  of  the  Coconino  Plateau  in  the  area  is  semi-arid  ; 
often  no  rain  will  fall  for  a  month  at  a  time.  The  precipi¬ 
tation  is  greatest  in  the  winter  months  and  in  the  month  of 
July.  Within  the  canyon  much  of  the  rainfall  evaporates 
before  it  reaches  the  greater  depths,  so  that  the  climate  of  the 
lower  part  of  the  canyon  is  more  arid  than  that  of  the  south¬ 
ern  plateau. 

Powell  Plateau,  whose  higher  eastern  portion  has  an  altitude 
equal  to  that  of  the  Kaibab  rim,  and  wliese  lower  western  por¬ 
tion  has  an  altitude  equal  to  that  of  the  Coconino,  has  an  inter¬ 
mediate  climate.  Its  situation  as  an  island  in  the  canyon  serves 
to  moderate  the  cold  in  winter,  for  the  warm  air,  rising  out  of 
the  deep  canyons  that  surround  it,  acts  as  a  radiator :  the  snow 
does  not  accumulate  so  deeply  on  the  eastern  end  as  on  the 
Kaibab ;  and  on  the  western  end  does  not  accumulate  at  all. 
Its  surface  is  a  resort  in  winter  for  the  game  and  wild  horses 
that  are  driven  out  of  the  Kaibab  by  the  snow.  Its  exposed 
position  subjects  it  to  violent  gales  of  wind  at  all  times  of  the 
year.  The  higher  eastern  end  receives  an  abundant  rainfall, 
while  the  lower  western  end  is  semi-arid. 

The  variation  of  the  flora  in  the  Shinumo  area  is  as  great  as 
that  of  the  climate.  The  surface  of  the  Kaibab  Plateau  is 
covered  with  a  magnificent  open  forest  of  yellow  pine  ;  the 
trees  grow  large  and  far  apart  and  the  ground  is  free  from 
undergrowth,  giving  its  surface  the  aspect  of  a  great  park  ; 
Englemann  spruces  grow  on  the  north  slopes  of  the  washes, 
and  cottonwoods,  aspens,  and  scrub  oaks  in  their  bottoms ;  a 
minor  flora  of  flowering  plants,  exceedingly  rich  in  species, 
covers  the  floor  of  the  forest.  The  flora  of  the  plateau  surface 
of  the  south  rim  of  the  canyon  differs  completely  from  that 
of  the  Kaibab  ;  it  is  covered  with  a  forest  of  gnarled  and 
stunted  trees  of  juniper  and  pinon,  with  here  and  there  a  buck¬ 
brush  bush  ;  the  trees  never  form  thickets,  but  grow  wide 
apart ;  while  the  open  stretches  are  covered  with  sage  brush 
and  mormon  tea,  with  occasional  cactus,  mescal,  and  plants  of 
the  century  family.  This  difference  between  the  floras  of  the 
north  and  south  rim  is  due  to  the  differences  in  precipitation 
and  temperature,  which  vary  directly  with  the  altitude.  For 
this  reason  the  floras  of  the  plateaus  furnish  an  almost  unfail¬ 
ing  index  of  the  elevation.  This  is  beautifully  shown  on  the 


380  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

southwestward-sloping  surface  of  Powell  Plateau,  the  whole 
eastern  half  of  which  lies  at  an  elevation  of  from  7000  to  7500 
feet  and  is  covered  with  the  open  pine  forest  characteristic  of 
the  Ivaibab.  At  about  7000  feet  the  character  of  the  flora 
changes,  and  passes  into  the  gnarled  and  stunted  forests  of  juni¬ 
per  and  pinon  characteristic  of  the  southern  plateau  across  the 
canyon. 

Within  the  canyon  itself  the  variation  in  the  flora  is  just  as 
great,  and  is  again  an  index  of  the  elevation. 

The  flora  of  the  Esplanade  platform,  a  thousand  feet  below 
the  south  rim,  consists  of  stunted  bushes  of  juniper  and  pinon 
with  grease  wood  as  the  ground  bush  in  place  of  the  sagebrush 
of  the  Coconino  Plateau.  The  cactus,  mescal,  and  plants  of  the 
century  family  are  present  in  greater  abundance  than  on  the 
plateau,  but  in  less  abundance  and  in  more  stunted  develop¬ 
ment  than  in  the  bottom  of  the  canyon.  This  is  due  to  the  fact 
that  the  Esplanade  level  is  within  reach  of  the  winter  snows 
and  frosts. 

The  flora  of  the  Tonto  platform,  three  thousand  feet  below 
the  south  rim,  and  of  all  the  interior  of  the  canyon  below  the 
Red  Wall,  is  the  flora  of  a  hot  and  arid  desert  in  its  most 
characteristic  form.  The  dominant  plants  are  the  greasewood 
bush,  the  mormon  tea,  and  the  cactus.  The  mescal  and  the 
plants  of  the  century  family  here  attain  their  greatest  develop¬ 
ment  and  size.  The  cacti  are  particularly  rich  in  species. 
Every  plant  in  the  flora  is  either  prickly  or  aromatic  ;  leaf  sur¬ 
faces  are  reduced  to  a  minimum ;  devices  for  storing  water 
attain  the  greatest  perfection ;  and  the  dominant  color  is  a 
somber  gray.  The  somber  colors  and  the  reduction  of  leaf 
surface  are  apt  to  deceive  the  observer,  both  in  regard  to  the 
richness  of  the  flora  in  species  and  the  abundance  of  plant 
life,  which  is  far  greater  than  one  would  suspect.  The  only 
tree  is  the  screw-mesquite,  which  grows  in  the  beds  of  those 
washes  that  contain  living  or  intermittent  streams. 

The  vegetation  in  the  bottoms  of  those  canyons  of  the  north 
side  in  the  Shinuino  Amphitheater  which  contain  living  streams 
is  beautiful  beyond  description,  and  in  refreshing  contrast  to 
the  desert  flora  of  the  Tonto  platform.  Tall  cottonwoods 
grow  in  the  lower  canyons ;  the  walls  are  hung  with  maiden¬ 
hair  fern  in  the  shady  places ;  and  willow  thickets  border  the 
stream.  Grass  grows  on  the  banks  where  there  is  soil. 
Higher  up  in  the  canyons,  oaks,  maples,  and  other  deciduous 
trees  come  in,  and  often  beds  of  tall  rushes.  The  most 
characteristic  bush  of  these  upper  north-side  canyons  is  the 
manzanita,  which  does  not  grow  on  the  south  side  of  the 
Grand  Canyon. 


The  Shinumo  Area. 


381 


General  Geology. 

Archean —  Vishnu  Schist. 

Name. — The  name  “  Vishnu  Terrane  ”  has  been  given  by 
Walcott  (Walcott,  c)  to  the  fundamental  crystalline  com¬ 
plex  of  the  Grand  Canyon  region  that  underlies  the  unaltered 
sedimentary  rocks  of  Algonkian  age,  and  is  separated  from 
them,  as  well  as  from  the  overlying  Cambrian,  by  a  profound 
unconformity.  The  type  locality  is  situated  on  the  Colorado 
River  thirty  miles  east  of  the  Shinumo  area  at  the  base  of  one 
of  the  great  buttes  called  “Vishnu’s  Temple,”  from  which  lie 
derives  the  name. 

I) istrihution  in  the  Kaibab  division. — The  rocks  of  the 
Vishnu  series  are  exposed  continuously  for  a  distance  of  over 
forty  miles  within  the  Kaibab  division  of  the  Grand  Canyon  in 
the  bed  of  the  Colorado  River  and  in  the  walls  of  its  gorge 
which  is  cut  beneath  the  Tonto  platform.  The  Shinumo  area 
is  at  the  western  end  of  this  long  exposure.  The  eastern  limit 
of  the  exposure  is  determined  by  the  appearance  of  great 
thicknesses  of  overlying  strata  of  the  Grand  Canyon  series 
dipping  northeastward  into  the  bed  of  the  river,  in  the  classic 
locality  described  by  Walcott.  The  western  limit  of  the 
exposure  is  likewise  defined  by  a  structural  cause  :  three  miles 
west  of  the  Shinumo  area  a  southward  bend  of  the  river 
causes  it  to  flow  parallel  to  the  gentle  southwestward  dip  of 
the  Paleozoic  rock  system  of  the  canyon  wall,  carrying  the  bed 
of  the  stream  up  out  of  the  schists  and  into  the  basal  Tonto 
sandstone  of  the  Cambrian. 

Occurrence  and  distribution  in  the  Shinumo  area. — The 
Vishnu  schists  are  exposed  for  a  distance  of  three  miles  in 
the  gorge  of  the  Muav-Flint  Creek  Canyon.  They  are  exposed 
in  the  bed  of  the  Colorado  river  throughout  its  course  across 
the  Shinumo  area.  Alone;  the  south  bank  of  the  river  for  a 
distance  of  four  miles,  and  along  the  north  bank  for  a  distance 
of  seven  miles,  in  the  eastern  half  of  the  area,  sediments  of 
the  Grand  Canyon  series  intervene  between  the  Vishnu  schists 
and  the  Tonto  sandstone  of  the  basal  Cambrian.  Elsewhere 
the  schists  are  exposed  without  break  in  the  walls  of  the  river 
gorge  beneath  the  Tonto  sandstone. 

So  detailed  study  of  these  rocks  was  made  in  the  Shinumo 
area  beyond  the  general  location  and  determination  of  the  vari¬ 
ous  types  represented,  since  the  limited  extent  of  the  expo¬ 
sures  precludes  the  possibility  of  unraveling  any  general 
Archean  structure  from  the  study  of  this  area  alone.  They 
are  here  a  metamoiqfiiic  complex  of  quartz,  mica,  and  horn¬ 
blende  schists,  invaded  by  a  batholithic  mass  of  quartz-diorite, 
and  injected  by  veins  of  pegmatite  and  aplite. 


382  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

Five  main  types  of  rock  are  found  in  this  complex  within 
the  area : 

The  first  type  is  a  quartz  schist,  with  graduations  into  mica 
schist.  It  comprises  the  greater  part  of  the  Vishnu  schists 
that  are  exposed  in  the  gorge  of  the  Colorado  River  west  of 
the  cable  crossing  at  Bass  Ferry.  It  is  also  exposed  in  the 
bed  of  Muav  Canyon,  and  in  the  bed  of  Flint  Creek  above  the 
junction  with  the  Shinumo. 

The  second  type  is  a  quartz  schist  with  gradations  into 
quartz-hornblende  schist.  It  is  exposed  in  that  part  of  the 
Muav-Flint  Creek  Canyon  that  is  occupied  by  the  living  stream 
of  the  Shinumo,  grading  both  eastward  and  westward  into  the 
quartz-mica  schist. 

The  third  type  is  a  hornblende  schist.  It  occurs  in  one 
small  outcrop  about  200  feet  wide,  on  the  east  side  of  “Fault 
wash,’’  a  dry  wash  which  joins  the  Shinumo  just  below  the 
mouth  of  Muav  Canyon,  and  is  sharply  bounded  on  both  sides 
by  the  quartz-mica  schist. 

The  fourth  type  of  rock  is  a  quartz-diorite.  So  far  as 
observed,  it  constitutes  all  of  the  Vishnu  series  of  the  Shinumo 
area  that  is  exposed  in  the  “  granite  gorge  ”  of  the  Colorado 
River  east  of  the  cable  crossing.  Its  western  contact  is  well 
defined.  The  eastern  limit  of  the  exposure  was  not  located. 

The  fifth  type  of  rock  is  a  granitic  pegmatite  which  occurs 
in  dikes  that  cut  all  the  rock  types  of  the  Vishnu.  These 
dikes  occur  in  two  generations.  The  older  generation  is  folded 
with  the  schists.  The  younger  generation  occurs  in  a  great 
network  or  mesh  of  dikes  cutting  both  the  quartz-diorite  and 
the  schists. 

The  Vishnu  schists  have  a  typical  schistose  structure.  The 
planes  of  scliistosity  have  a  dip  that  seldom  departs  greatly 
from  the  vertical.  The  dominant  trend  of  the  scliistosity  is 
northeasterly,  but  varies,  from  place  to  place.  Locally,  the 
schists  are  much  twisted  and  contorted. 

Lithology. — The  less  micaceous  phase  of  the  quartz-mica 
schist  has  a  dark  grey  color  with  a  greenish  tinge.  The  cleav¬ 
age  is  imperfect  and  the  surfaces  have  a  satin-like  luster. 
The  texture  is  fine-grained  and  the  unaided  eye  can  distinguish 
no  mineral  constituents  except  quartz.  The  microscope  shows 
the  rock  to  be  composed  almost  entirely  of  fine  interlocking 
grains  of  quartz,  with  occasional  small  flakes  of  white  mica, 
arranged  in  parallel  lines.  In  the  extreme  quartzose  phase 
the  amount  of  mica  is  almost  negligible,  just  enough  being 
present  to  impart  a  satin-like  luster  to  the  rock. 

The  micaceous  phase  of  the  schist  has  a  slight  grey  color 
with  either  a  pinkish  or  greenish  tinge  on  the  fresh  fracture. 
Where  the  rock  is  weathered,  the  color  becomes  red.  The 


The  Shinumo  Area. 


383 


cleavage  is  rather  distinct.  The  texture  ranges  from  fine  to 
coarse.  To  the  unaided  eye  both  the  quartz  and  mica  are  visi¬ 
ble.  Under  the  microscope  the  rock  is  seen  to  be  composed 
of  interlocking  grains  of  quartz,  with  an  almost  equal  amount 
of  mica.  The  mica  flakes  are  arranged  in  parallel  lines.  The 
greater  part  of  the  mica  is  muscovite,  with  occasional  flakes  of 
brown  biotite. 

Locally  the  schist  is  garnetiferous,  and  in  one  locality  tour¬ 
maline  was  observed.  All  gradations  between  the  quartzose 
and  the  micaceous  phases  occur,  but  in  no  instance  does  the 
mica  exceed  the  quartz  in  quantity. 

The  quartz-hornblende  schist  is  dark  green  in  color.  The 
cleavage  is  imperfect,  and  the  texture  fine-grained  and  uni¬ 
form.  The  rock  is  dense  and  hard.  The  mineral  constituents 
cannot  usually  be  distinguished  in  the  hand  specimen  without 
the  aid  of  the  lens.  Under  the  microscope  the  rock  is  seen  to 
consist  of  about  equal  proportions  of  quartz  and  green  horn¬ 
blende.  The  quartz  is  present  in  interlocking  grains.  The 
hornblende  tends  to  form  automorphic  crystals  whose  longer 
axes  have  a  roughly  parallel  arrangement.  No  other  minerals 
were  present  in  the  slides  examined.  In  some  phases  of  the 
rock  the  quartz  exceeds  the  hornblende  in  amount. 

The  hornblende  schist  is  dark  green  in  color.  It  is  a  soft 
rock,  considerably  disintegrated,  and  crumbling  under  the 
hammer.  The  texture  is  coarse  granular.  Megascopically  the 
rock  appears  to  consist  almost  entirely  of  dark  green  horn¬ 
blende.  No  schistose  structure  is  observable  in  the  mass. 
The  microscope  shows  the  rock  to  consist  almost  wholly  of 
large  crystals  of  green  hornblende  in  all  stages  of  alteration. 
A  small  amount  of  interstitial  quartz  occurs.  The  quartz 
granules  are  strung  out  in  a  roughly  parallel  fashion.  The 
rock  is  badly  altered  and  the  thin  section  is  unsatisfactory. 

The  quartz-diorite  found  in  the  river  gorge  eastward  from 
the  cable  crossing  is  a  coarse-granular  rock  of  typical  granitic 
texture.  It  is  a  hard,  resistant  rock,  which  tends  to  weather 
into  roughly  angular  blocks,  a  feature  that  distinguishes  it  in 
the  mass  from  the  exposures  of  the  schists.  The  minerals  visi¬ 
ble  to  the  unaided  eye  are  white  striated  feldspar,  dark  horn¬ 
blende,  and  glistening  black  biotite.  The  color  of  the  rock  is 
dark  grey  and  the  appearance  is  remarkably  fresh.  The  tex¬ 
ture  is  uniform  throughout  the  exposures  observed,  and  the 
rock  is  apparently  without  contact  modifications.  No  ten¬ 
dency  to  gneissoid  banding  was  observed. 

Under  the  microscope  it  is  seen  to  be  a  coarse-granular  rock 
of  granitic  texture.  The  dominant  mineral  constituents  are 
plagioclase  and  common  hornblende.  The  plagioclase  ranges 
from  oligoclase  to  labradorite.  Microcline,  orthoclase,  and 


384  Noble — Geology  of  the  Grand  Canyon,  Arizona. 

quartz  are  present  in  about  equal  proportions,  but  their  total 
amount  does  not  equal  that  of  the  plagioclase.  Brown  biotite 
is  present  in  somewhat  less  quantity  than  the  hornblende. 
Titanite  and  magnetite  occur  as  accessories.  Occasionally  the 
quartz  is  poikilitic  in  the  orthoclase.  The  feldspathic  and  fer- 
romagnesian  minerals  are  present  in  about  equal  proportions. 
If  it  were  not  for  the  preponderance  of  the  plagioclase  the 
rock  might  be  classed  as  a  quartz-monzonite  or  grano-diorite. 
It  is  probably  best  classed  as  a  quartz-diorite  with  a  monzo- 
nitic  aspect.  The  microscope  reveals  no  cataclastic  structure 
nor  other  evidence  of  dynamic  action,  and  the  minerals  are 
fresh  and  unaltered. 

Th e  pegmatites  are  pink  in  color  and  usually  very  coarse  in 
texture.  They  are  composed  chiefly  of  quartz  and  pink  ortho¬ 
clase.  Some  of  the  dikes  carry  large  crystals  of  silvery- white 
mica.  No  other  minerals  were  noted.  The  dikes  usually 
exhibit  the  typical  comb-structure  inward  from  the  walls  and 
the  graphic  arrangement  of  the  quartz  and  feldspar.  Along 
the  walls  of  some  of  the  dikes  the  texture  becomes  aplitic. 

Origin. — There  is  no  clear  evidence  in  the  Shinumo  area  by 
which  the  original  character  of  the  schists  of  the  Vishnu 
series  can  be  determined.  There  is  no  evidence  of  a  banding 
that  can  be  clearly  referred  to  original  sedimentary  bedding, 
nor  is  there  evidence  of  any  original  clastic  texture.  The 
mineralogical  composition,  however,  suggests  a  sedimentary 
origin  for  the  quartz  schists  of  the  mica  and  hornblende  type  : 
either  might  have  resulted  from  the  regional  metamorphism  of 
an  arkose  sandstone  or  shale.  According  to  the  lesser  or 
greater  abundance  of  iron  in  the  original  sediments  the  rock 
would  assume  the  mica  or  hornblende  type  of  quartz  schist. 
Certainly  the  great  preponderance  of  quartz  as  a  constituent 
of  these  rocks  would  seem  to  weigh  against  an  igneous  origin 
in  the  balance  of  probability.  The  causes  operating  to  pro¬ 
duce  the  schists  in  their  present  aspect  are  conceived  to  be 
the  processes  connected  with  regional  metamorphism,  namely, 
subsidence  and  deep  burial,  subsequent  folding  and  mashing, 
and  a  slow  recrystallization  in  process  of  time. 

The  original  character  of  the  hornblende  schist  described  as 
occurring  in  a  narrow  outcrop  between  the  schists  is  not  clear. 
The  fact  that  the  rock  consists  of  little  else  than  hornblende 
suggests  that  it  originally  constituted  an  igneous  rock  of  a 
basic  type.  The  fact  that  the  microscope  discloses  a  schistose 
structure  is  evidence  that  the  rock  is  at  least  earlier  in  age 
than  the  period  of  regional  metamorphism  that  was  responsible 
for  the  present  structural  and  mineralogical  character  of  the 
enclosing  schists. 

The  origin  of  the  quartz-diorite  is  reasonably  clear.  Since 


The  Shinumo  Area. 


385 


it  is  a  coarse-granular,  igneous  rock  of  plutonic  aspect  occur¬ 
ring  over  a  large  area,  clearly  cutting  the  schists,  and  showing 
no  textural  modifications  at  the  contact,  it  is  concluded  that 
it  represents  a  deep-seated  igneous  invasion  of  considerable 
size,  of  the  type  known  as  a  “  batholith.”  Because  of  the 
unaltered  character  of  the  rock  and  the  absence  of  any  gneis- 
soid  or  cataclastic  structure,  it  is  argued  that  the  batholithic 
invasion  took  place  at  a  time  later  than  the  period  of  regional 
metamorpliism  that  produced  the  recrystallization  and  schistos- 
ity  of  the  enclosing  schists.  The  question  may  arise  as  to 
whether  the  invasion  of  the  batholith  was  not  of  itself  a  par¬ 
tially  operative  cause  in  producing  this  recrystallization  and 
schistosity.  The  field  evidence  is  against  such  a  conclusion  : 
there  is  no  gradation  in  the  schist  away  from  the  contact, 
either  in  texture  or  in  mineral  composition,  nor  are  the  planes 
of  schistosity  parallel  to  the  contact. 

The  older  pegmatite  dikes  are  folded  intimately  with  the 
schists.  Their  injection  may  have  either  preceded  or  accom¬ 
panied  the  regional  metamorpliism. 

The  younger  pegmatite  dikes  cut  both  the  schists  and  the 
quartz-diorite.  Where  they  cut  the  schists  they  break  cleanly 
across  the  schistosity.  The  injection  of  these  dikes  is  the  latest 
visible  event  in  the  igneous  activity  of  Archean  time  within 
the  area. 

Age  and  correlation. — The  rocks  of  the  Yishnu  series  are 
rocks  which,  in  the  light  of  present  knowledge,  can  only  be 
conceived  to  have  acquired  their  present  character  at  great 
depths  beneath  the  earth’s  surface  in  what  is  technically  known 
as  the  u  zone  of  towage.’’  It  is  therefore  evident  that  the 
unconformity  which  separates  them  from  the  overlying  Algon- 
kian  sediments  of  the  Grand  Canyon  series  represents  a  vast 
amount  of  erosion  and  the  consequent  lapse  of  an  enormous 
interval  of  time, — an  interval  vastly  greater  even  in  events 
than  that  represented  by  the  profound  unconformity  which 
separates  the  succeeding  Grand  Canyon  series  from  the  over- 
lying  Paleozoic.  The  Yishnu  schists  are  therefore  assigned  to 
the  Archean  in  the  usage  of  the  United  States  Geological 
Survey ;  but  whether  as  a  group  or  a  complex  is  as  yet  unde¬ 
cided.  It  seems  likely,  as  stated  by  Bansome  ( a ,  p.  21),  that 
they  are  to  be  correlated  with  the  Pinal  Schist  of  the  Globe 
and  Bisbee  regions,  and  “  present  somewhat  different  aspects 
of  the  fundamental  crystalline  complex  of  Arizona/1 

Up  to  the  present  time  no  detailed  study  of  these  rocks  has 
been  made  in  the  Grand  Canyon  region,  and  their  internal 
structural  relations  are  unknown.  It  is  not  unlikely  when 
such  a  study  is  made  of  their  exposures  in  the  Kaibab  division 
of  the  canyon,  in  the  Shiv  wits  division  to  the  west,  and  along 

Am.  Jour.  Sci. — Fourth  Series,  Vol.  XXIX,  No.  173. — May,  1910. 

26 


498  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

Colorado  in  the  eastern  end  of  the  Kaibab  division  and  the 
mouth  of  Tapeats  Creek  some  80  miles  below  in  the  eastern 
end  of  the  Kanab  division,  where  the  strata  of  the  Grand 
Canyon  Series  are  exposed  between  the  crystalline  schists  of 
the  Archean  and  the  basal  Tonto  sandstone  of  the  Cambrian. 
The  location  of  these  exposures  is  shown  on  the  map  accom¬ 
panying  this  article.  Five  of  the  localities  are  within  the 
Kaibab  division ;  the  sixth  is  within  the  Kanab. 

The  first  of  these  localities  is  the  classic  area  below  the 
mouth  of  the  Little  Colorado  described  by  Walcott.  This  is 
the  largest  areal  exposure  of  these  rocks  in  the  Grand  Canyon, 
and  includes  both  the  Unkar  and  Chuar  groups. 

The  second  locality  lies  five  miles  west  of  the  first  at  the 
head  of  the  inner  gorge  of  Clear  Creek  on  the  north  side  of 
the  Colorado  river  within  the  depths  of  the  Ottoman  Amphi¬ 
theater.  The  exposure  is  limited  to  less  than  a  square  mile. 
It  comprises  a  small  portion  of  the  basal  Unkar  and  is  struc¬ 
turally  a  unit  with  the  first  locality. 

The  third  locality  lies  along  the  north  side  of  the  Colorado 
river  at  the  mouth  of  Bright  Angel  Creek  opposite  the  railroad 
terminus  and  hotels  of  the  Saute  Fe  Kailroad.  About  1000 
feet  of  the  basal  portion  of  the  Unkar  group  are  there  repre¬ 
sented  and  the  areal  extent  of  the  exposure  is  about  three 
square  miles.  This  locality  has  been  briefly  described  by 
Kan  so  me  (Kansome,  a).  It  lies  about  10  miles  west  of  the 
type  locality. 

The  fourth  locality  comprises  a  limited  exposure  of  basal 
Unkar  strata  which  lies  in  the  depths  of  the  Hindu  Amphi¬ 
theater  on  the  north  side  of  the  Colorado  river  about  three 
miles  up  Crystal  Creek  from  its  mouth.  It  is  situated  some 
20  miles  west  of  the  type  locality.  The  areal  extent  of  the 
exposure  is  about  one  square  mile.  It  is  as  yet  undescribed. 

The  fifth  locality  lies  about  the  month  of  Shinumo  Creek 
about  30  miles  west  of  the  exposures  of  the  type  area.  It  com¬ 
prises  about  12  square  miles  in  areal  exposure  and  represents 
nearly  the  entire  thickness  of  the  Unkar  group.  It  is  hitherto 
un described  in  geological  literature  and  is  the  subject  of  the 
succeeding  pages  of  this  article. 

The  sixth  locality  is  situated  on  the  Colorado  river  just 
above  the  mouth  of  Tapeats  Creek  in  the  eastern  end  of  the 
Kanab  division  of  the  Canyon,  about  12  miles  northwest  of 
the  mouth  of  the  Shinumo  in  a  direct  line  and  about  25  miles 
down  the  river  in  its  actual  course.  The  length  of  the  expo¬ 
sure  is  about  three  miles  in  the  bed  of  the  river  and  in  its 
narrow  gorge  beneath  the  Tonto  sandstone.  About  4,000  feet 
of  the  basal  portion  of  the  Unkar  group  are  exposed,  striking 
H.W.-S.E.  and  dipping  about  15°  N.E.  This  locality  is 


The  Shinumo  Area. 


499 


structurally  a  unit  with  the  exposures  of  the  Shinumo  area. 
It  is  un mentioned  in  geological  literature. 

In  the  western  end  of  the  Kanab  division  in  a  section  across 
the  Grand  Canyon  at  the  foot  of  Toroweap  valley,  50  miles 
west  of  the  Shinumo  area,  Dutton  figures  “  rocks  of  Silurian 
and  Archean  unconformable”  in  the  bed  of  the  river  beneath 
the  basal  Tonto  sandstone  of  the  Cambrian  (Dutton,  a ,  p.  88). 
It  is  probable  that  these  u  Silurian”  rocks  there  represent  the 
Grand  Canyon  series. 

Whether  these  rocks  appear  at  places  in  the  Sliivwits 
division  between  the  Vishnu  schist  and  the  Tonto  sandstone 
is  not  at  present  known. 

Stratigraphic  position ,  structiire ,  and  distribution  in  the 
Shinumo  Area. — Two  unconformities  determine  the  strati¬ 
graphic  position  of  the  sediments  of  the  Grand  Canyon  series 
in  the  Shinumo  Area  :  they  are  separated  from  the  underlying 
Vishnu  schists  of  the  Archean  by  a  profound  unconformity 
which  represents  a  base-leveled  surface  of  erosion,  and  from 
the  overlying  Tonto  sandstone  of  the  Cambrian  by  an  uncon¬ 
formity  which  represents  a  similar  base-leveled  surface  above. 

The  strata  of  the  Grand  Canyon  series  here  constitute  a 
wedge-shaped  mass  whose  apex  lies  along  the  south  side  of 
the  Colorado  river  parallel  to  its  northward  course  in  this 
part  of  the  area.  The  mass  as  a  whole  constitutes  one  great 
tilted  block,  which  in  turn  consists  of  a  great  number  of  minor 
tilted  and  rotated  blocks  pitching  at  successively  greater  angles 
northeast  away  from  the  apex  of  the  wredge,  until  at  a  distance 
of  three  miles  from  the  Colorado  river  the  whole  mass  is 
dropped  by  a  profound  fault  which  brings  up  the  under¬ 
lying  Vishnu  schist  from  a  great  depth, — a  structure  which 
strikingly  resembles  that  of  the  area  of  similarly  faulted 
Triassic  blocks  of  the  Connecticut  valley.  The  strike  of  the 
strata  is  K.  40°  W.  The  dips  are  variable :  in  general  the 
strata  of  the  fault  blocks  near  the  apex  of  the  wedge  dip 
10°-15°  N.E. ;  near  the  center  of  the  wedge  the  dips  average 
25°  N.E. ;  while  in  proximity  to  the  line  of  the  great  limiting 
fault  on  the  northeast  they  are  completely  reversed  by  the 
“drag”  along  the  fault  plane.  The  truncation  of  this  pre- 
Cambrian  structure  by  the  unconformity  at  the  base  of  the 
Tonto  sandstone  is  absolute. 

The  great  pre-Cambrian  fault  that  limits  the  wedge  upon 
the  northeast  represents  the  exposure  in  the  basement  rocks  of 
the  line  of  displacement  of  the  West  Kaibab  fault  and  displays 
in  the  most  spectacular  manner  a  phenomenon  analogous  to  that 
described  by  Walcott  upon  the  line  of  the  East  Kaibab  mono¬ 
cline  (Walcott,  a).  Upon  the  line  of  the  ancient  fault  in  the 
Shinumo  area  two  later  displacements  have  taken  place  after 


500  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

the  deposition  of  the  entire  Paleozoic  series  of  the  Canyon 
wall  and  probably  later  strata.  The  first  of  these  is  a  mono- 
clinal  flexure  which  reverses  the  throw  of  the'  pre-Cambrian 
fault,  while  the  second  is  a  still  more  recent  fault  super¬ 
imposed  upon  the  line  of  the  monoclinal  flexure. 

The  strata  of  the  Grand  Canyon  series  are  exposed  beneath 
the  Tonto  sandstone  in  all  that  part  of  the  inner  gorge  of  the 
Muav-Flint  Creek  canyon  which  is  on  the  south  side  of  the  great 
pre-Cambrian  fault, — a  distance  of  about  three  miles.  They 
are  exposed  for  three  miles  in  the  gorge  of  the  Shinumo 
Canyon ;  for  seven  miles  along  the  north  side  of  the  Colorado 
river ;  and  in  all  the  inter-canyon  valleys  within  that  distance 
which  are  eroded  below  the  base  of  the  Tonto  sandstone.  The 
two  largest  of  these  inter-canyon  valleys  are  the  u  East  Wash,” 
a  mile  east  of  the  Shinumo,  and  the  “  Asbestos  Canyon,”  three 
miles  to  the  west. 

The  gorge  of  the  Colorado  river  has  everywhere  been 
trenched  to  a  depth  sufficient  to  expose  the  Vishnu  schists 
along  the  river  beneath  the  overlying  strata  of  the  Grand 
Canyon  series.  This  is  due  to  the  fact  that  the  course  of  the 
river  lies  close  along  the  southern  apex  of  the  wedge. 

The  exposures  on  the  south  side  of  the  river  are  more  lim¬ 
ited,  due  to  the  thinning  out  of  the  wedge  in  that  direction 
and  the  lack  of  inter-canyon  valleys  trenched  beneath  the  Tonto 
sandstone.  The  strata  are  exposed  for  two  miles  above  and 
one  mile  below  a  point  opposite  the  mouth  of  the  Shinumo. 

A  southwestward  bend  in  the  river  in  the  western  part  of 
the  area  carries  it  beyond  the  apex  of  the  wedge,  below  which 
point  the  Tonto  sandstone  caps  the  Vishnu  schists  which  lie  in 
the  river  gorge.  Southeastward  up  the  river,  in  the  eastern 
part  of  the  area,  a  similar  relation  obtains. 

The  hard  middle  members  of  the  Unkar  resisted  the  erosion 
which  preceded  the  deposition  of  the  Cambrian  sandstone  and 
stood  as  an  island  in  the  Tonto  sea. 

This  long  monadnock  of  quartzite  runs  across  the  area  in  a 
JVW.-S.E.  direction  parallel  to  the  general  strike  of  the  strata 
of  the  wedge,  and  a  narrow  outcrop  of  these  quartzites  is  ex¬ 
posed  along  the  Tonto  platform  on  the  north  side  of  the  river 
just  at  the  base  of  the  Kedwall  clifT,  running  for  a  distance  of 
about  five  miles  beyond  the  main  areal  exposures  about  the 
mouth  of  the  Shinumo  and  uniting  with  the  limestones  of  the 
upper  Tonto  group  to  form  the  lower  part  of  the  great  cliff  of 
Redwall  limestone.  In  the  eastern  part  of  this  exposure  the 
quartzite  monadnock  projects  700  feet  above  the  base  of  the 
Tonto  sandstone.  Westward  from  the  exposures  about  the 
Shinumo  the  prolongation  of  the  monadnock  along  the  strike 
of  the  quartzites  exposes  them  in  a  narrow  outcrop  upon  the 
Tonto  platform  one  mile  west  of  the  Shinumo. 


The  Shinumo  Area. 


501 


The  total  areal  exposure  of  the  Grand  Canyon  series  in  the 
Shinumo  area  is  about  12  square  miles. 

In  following  up  the  Shinumo  from  its  mouth  to  the  point 
where  it  leaves  the  lateral  gorge  of  the  Muav-Flint  Creek  can¬ 
yon,  a  traverse  is  made  of  the  total  exposed  thickness  of  the 
Unkar  group  from  the  unconformity  at  the  base  to  the  highest 
member  that  is  limited  by  the  profound  fault  on  the  northeast. 
There  can  hardly  be  a  more  magnificent  illustration  of  details 
of  geological  structure  than  is  here  revealed.  Along  the  entire 
western  side  of  the  Shinumo  canyon,  in  cliff  faces  a  thousand 
feet  above  the  bed  of  the  stream,  is  displayed  every  detail  of 
the  structure  beneath  the  basal  Tonto  sandstone.  Westward 
down  the  Colorado  river  the  intersection  of  the  two  great 
unconformities  forming  the  apex  of  the  wedge  is  seen  in  the 
cliff  face  above  the*  river  bank,  below  which  point  the  river 
narrows  in  its  somber  gorge  in  the  Vishnu  schists.  From  here 
northeastward,  in  the  cliff  faces  along  the  western  wall  of  the 
Shinumo  canyon,  bed  after  bed  of  the  Unkar  strata  appears, 
wedging  out  southwestward  beneath  the  plane  of  the  uncon¬ 
formity  beneath  the  Tonto  sandstone.  Every  detail  of  the 
successive  fault  blocks  of  the  great  wedge  is  clearly  shown, — 
their  increasing  tilt  northeastward,  the  dips  of  the  fault  planes 
that  bound  them,  and  the  occasional  down-dropped  wedges. 
Above  runs  the  plane  of  the  pre-Tonto  unconformity,  revealing 
in  cross  section  the  monadnock  in  this  peneplain  which  existed 
as  a  rocky  island  during  the  inroads  of  the  Tonto  sea,  the  debris 
from  its  wave-cut  cliffs  being  incorporated  and  preserved  to  the 
minutest  detail  in  the  Tonto  sandstone.  In  the  background, 
bed  above  bed  in  conformable  succession,  lies  the  horizontal 
Paleozoic  section  in  the  wall  of  the  mile-deep  Canyon.  After 
traversing  a  thickness  of  5800  feet  of  Unkar  strata  in  a  distance 
of  three  miles  dipping  northeastward  into  the  bed  of  the  stream, 
the  traveler  crosses  the  line  of  the  great  pre-Cambrian  fault  of 
more  than  5800  feet  and  comes  once  more  into  the  Vishnu 
schists  on  the  farther  side  of  the  Muav-Flint  Creek  canyon. 

Here  again  is  an  instance  of  the  simplicity  with  which  the 
geological  structure  is  revealed  in  this  wonderful  country. 
Along  the  wTiole  northern  wall  of  the  lateral  gorge  lie  the 
Vishnu  schists  below  the  Tonto  sandstone  cliff.  On  the  south¬ 
ern  side,  at  the  same  level,  lie  the  upper  sandstones  of  the 
Unkar,  their  beds  dragged  up  sharply  against  the  fault  line, 
which  lies  in  the  bed  of  the  stream.  The  whole  Paleozoic 
system  on  the  northern  side  of  the  gorge  has  been  dropped 
500  feet  by  the  torn  monocline  of  the  West-Kaibab  fault, 
reversing  the  throw  of  the  pre-Cambrian  fault  on  the  same 
line  in  the  basement  rocks.  Looking  westward  up  the  Muav 
.  Canyon,  the  beds  of  the  Paleozoic  are  seen  bending  down 


502  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 


against  the  fault  line  in  a  graceful  arc.  Far  up  the  canyon 
at  its  head  under  the  Muav  Saddle,  the  more  recent  fault  has 
reversed  its  throw  and  restored  the  throw  of  the  monoclinal 
flexure.  There  is  not  a  detail  of  this  structure  that  may  not 
he  seen  at  a  glance. 


Sediments  of  the  Unkae  Group. — Lithology. 

Preliminary  Outline. — The  pre-Cambrian  sedimentary  rocks 
of  the  Shinumo  area  represent  the  greater  part  of  the  Unbar 
group  of  the  Grand  Canyon  series  of  Algonkian  age.  The  upper, 
or  Clmar,  group  is  not  represented  in  the  area.  Although 
these  sediments  present  no  more  evidence  of  alteration  or  meta- 
morpliism,  aside  from  local  igneous  contact  phenomena,  than 
the  overlying  beds  of  the  Paleozoic,  they  are  destitute  of  fos¬ 
sils  or  decisive  evidence  of  life.  In  the  absence  of  fossils  the 
natural  basis  for  dividing  the  group  into  component  mem¬ 
bers  is  the  lithology.  On  this  basis  the  Unkar  group  in  the 
Shinumo  area  is  divisible  into  five  members,  succeeding  each 
other  in  conformable  stratigraphic  succession.  It  is  not  in¬ 
tended  that  the  importance  of  this  division  should  be  greatly 
emphasized.  Its  chief  value  lies  in  the  fact  that  it  furnishes  a 
basis  for  comparing  the  lithological  succession  of  the  Unkar 
group  in  this  area  with  that  in  the  type  locality  described  by 
Walcott  30  miles  to  the  east,  as  well  as  a  basis  for  distinguish¬ 
ing  in  a  broad  way  the  main  changes  in  the  physical  conditions 
under  which  these  sediments  were  laid  down. 

At  the  base,  resting  upon  the  profoundly  eroded  and  base- 
leveled  surface  of  the  metamorpliic  rocks  of  the  Vishnu  series, 


Tonto  Group 
Unconformity. 


4. 

3. 


Unkar  Group. 


Synopsis. 


Micaceous  shaly  sandstone  (exclusive 


of  36  feet  of  intrusive  rock) . .  2297 

Sandstone  and  quartzite _  1564 

Argillaceous  and  arenaceous  shale 
(exclusive  of  950  feet  of  intrusive 
rock) _ _ 580 


I  2.  Calcareous  shale  and  limestone _ _  335 

1.  Basal  conglomerate _ _  6 

i 

i 


Unconformity 
Vishnu  series. 


T  otal . . 


4782  ft. 


The  Shinumo  Area. 


503 


is  a  thin  conglomerate.  This  constitutes  the  basal  member  of 
the  Unkar.  Overlying  the  conglomerate  is  a  series  of  lime¬ 
stones  and  calcareous  shales.  These  grade  upward  into  argil¬ 
laceous  and  arenaceous  shales  which  are  intruded  by  a  thick 
sill  of  diabase,  and  are  succeeded  in  turn  by  great  thicknesses 
of  sandstone  and  quartzite.  The  uppermost  exposed  member 
of  the  group  in  the  area  is  a  thick  series  of  micaceous  slialy 
sandstones. 

It  has  been  shown  that  these  strata  lie  in  a  wedge-shaped 
mass  inset  in  the  Vishnu  schists,  and  that  this  wedge  is  com¬ 
posed  of  a  great  number  of  smaller  titled  fault  blocks.  It  is 
apparent  from  this  relation  that  nowhere  in  the  Shinumo  area 
can  the  thickness  be  measured  in  one  unbroken  section.  Since, 
however,  the  lithological  characters  of  the  strata  are  constant 
and  easily  recognized,  and  since  the  throw  of  the  faults  that 
bound  the  titled  blocks  seldom  exceeds  100  feet,  the  restoration 
of  a  section  showing  the  unbroken  sequence  is  not  a  matter  of 
great  difficulty. 

Detailed  sections  were  made  upward  from  the  base  of  the 
Unkar  through  each  fault  block  until  its  limiting  fault  was 
reached.  The  highest  bed  measured  was  then  located  in  the 
next  block  to  the  northeast,  and  the  measurement  resumed  at 
that  point.  Except  in  the  fifth,  or  highest  member  of  the 
group,  all  sections  were  measured  with  a  tape  along  the  nearly 
vertical  wall  faces  of  the  box  canyons  of  the  Shinumo  and 
other  wTashes  that  cut  across  the  strike  of  the  strata.  In  the 
fifth,  or  highest  member  of  the  group,  the  strong  drag  of  the 
great  fault  on  the  northeast  has  flexed  and  contorted  these 
slialy  sandstones  in  such  a  manner  that  accurate  measurement 
with  the  tape  alone  was  impossible.  Their  thickness  was 
computed  by  the  aid  of  trigonometric  formulae,  using  the  com¬ 
bined  data  afforded  by  the  use  of  the  tape,  the  topographic 
map,  and  observations  of  the  varying  strike  and  dip. 

The  section  incorporated  in  this  article  was  made  in  two 
places.  The  greater  part  of  the  total  thickness  was  measured 
in  a  traverse  up  the  Shinumo.  This  section  includes  all  the 
strata  above  the  diabase  sill  which  is  intruded  midway  in  the 
“arenaceous  and  argillaceous  shales”  which  comprise  the  third 
member  of  the  Unkar.  It  would  have  been  perfectly  possible 
to  make  a  complete  section  of  the  group  in  a  traverse  of  the 
entire  course  of  the  Shinumo  from  the  basal  unconformity  at 
the  mouth  of  the  creek  to  the  great  fault  three  miles  above, 
although  four  faults  cross  the  creek  between  its  mouth  and  the 
place  where  the  diabase  sill  dips  beneath  the  bed  of  the  stream. 
But  a  place  was  found  in  the  canyon  of  the  East  Wash  where 
all  the  strata  between  the  basal  unconformity  and  the  diabase 
sill  lie  in  a  continuous  unfaulted  section,  in  a  fault  block  that 


504  Noble — Geology  of  the  Grand  Canyon,  Arizona. 


is  tilted  about  10°  N.E.  The  section  of  the  basal  members  of 
the  group  was  measured  in  this  locality,  where  the  sequence  is 
unbroken  by  faulting. 


Detailed  Section. 

First  Member. — Basal  Conglomerate.  The  surface  rep¬ 
resented  by  the  upper  unconformity  that  separates  the  Unkar 
group  from  the  basal  Cambrian  is  a  striking  enough  example 
of  a  base-leveled  surface,  although  monadnocks  rise  in  places  to 
a  height  of  700  feet  above  the  base  of  the  Tonto  sandstone. 
But  the  surface  represented  by  the  lower  unconformity  that 
separates  the  Unkar  group  from  the  Vishnu  is  an  almost  per¬ 
fect  plane  :  nowhere  in  the  seven  linear  miles  exposed  in  the 
Shinumo  area  can  a  difference  in  relief  be  observed  that 
exceeds  20  feet.  The  depth  of  weathering  below  this  surface 
appears  to  be  slight,  in  spite  of  the  enormous  amount  of  rock 
that  has  been  removed,  and  the  weathering  appears  to  be  the 
result  of  physical  disintegration  rather  than  of  chemical 
decomposition. 

The  basal  conglomerate  is  an  arkose  conglomerate  varying 
in  thickness  from  1  to  6  feet  in  the  Shinumo  area.  It  is  com¬ 
posed  of  angular  or  subangular  fragments  of  the  rocks  of  the 
underlying  Vishnu  series,  cemented  by  a  matrix  of  red  arkose 
mud  which  usually  contains  small  fragments  of  pink  feldspar. 
Occasionally  the  matrix  contains  small  rounded  grains  of 
quartz. 

The  degree  of  induration  of  this  conglomerate  presents  all 
variations  from  a  hard,  dense,  siliceous  rock,  which  fractures 
across  matrix  and  enclosed  rock  fragments  alike,  to  an  easily 
disintegrated  rock  in  which  the  matrix  crumbles  away  from 
the  enclosed  fragments.  This  phenomenon,  however,  does  not 
depend  upon  original  cementation,  but  upon  metamorphic 
effects  produced  by  the  diabase  sill  that  is  intruded  in  the  over- 
lying  rocks,  the  degree  of  induration  depending  upon  how  far 
the  conglomerate  lies  below  the  contact  of  the  sill. 

The  matrix  is  usually  of  the  same  composition  everywhere 
in  the  area.  The  character  of  the  enclosed  fragments,  how¬ 
ever,  is  sharply  localized  by  the  character  of  the  underlying 
rock.  The  rock  which  underlies  the  conglomerate  in  the  East 
Wash  is  the  quartz-diorite  of  the  batholith  previously 
described.  For  three  feet  below  the  conglomerate  the  diorite 
is  weathered  along  the  joints  into  roughly  angular  blocks. 
These  joints  are  filled  with  the  red  arkose  material  of  the 
matrix.  Above  follows  a  layer  of  the  conglomerate  one  foot 
in  thickness,  composed  of  weathered  fragments  of  the  diorite 
cemented  with  the  red  arkose  material.  Then  follows  a  layer 
six  inches  thick  composed  of  small  rounded  quartz  pebbles  and 


The  Shinumo  Area. 


505 


fragments  of  cliert  of  the  same  character  as  that  contained  in 
the  overlying  limestones.  The  whole  is  cemented  with  the  red 
mud.  The  conglomerate  is  very  little  indurated  in  this  local¬ 
ity.  Although  the  contact  of  the  diorite  with  the  mica-scliists 
in  the  underlying  Vishnu  is  not  two  hundred  yards  distant, 
there  is  not  a  fragment  of  the  mica-schist  to  be  observed  in  the 
conglomerate. 

In  the  Asbestos  Canyon,  four  miles  to  the  west  of  the  East 
Wash,  the  underlying  rocks  are  mica  schists  and  veins  of 
quartz  and  pegmatite.  Here  the  Vishnu  schist  is  scarcely 
weathered  at  all  below  the  unconformity.  The  overlying 
conglomerate  is  6  feet  thick  and  consists  of  angular  fragments 
of  the  underlying  mica-schists,  fragments  of  pegmatitic  feldspar 
and  vein  quartz,  and  the  arkose  cement  described  above.  ,  The 
degree  of  induration  is  here  very  great,  and  the  rock  fractures 
across  the  grains  like  a  hard,  dense  quartzite.  This  is  due  to 
the  fact  that  the  lower  contact  of  the  diabase  sill  lies  only  150 
feet  above  this  basal  conglomerate  in  the  Asbestos  Canyon, 
while  in  the  locality  in  the  East  Wash  it  lies  550  feet  above. 

Two  important  features  characterize  the  basal  conglomerate 
of  the  Unkar  in  the  Shinumo  area ;  the  arkose  nature,  and  the 
lack  of  sorting  and  transportation  of  the  component  frag¬ 
ments. 

Second  Member. — Calcareous  Shales  and  Limestone.  The 
section  was  measured  on  the  west  side  of  the  canyon  of 
the  East  Wash.  This  and  all  the  following  sections  read  from 
the  base  upward,  a.  1  being  the  bottom  bed,  overlain  by  a.  2, 
etc. 


Charactek  Thickness 

a.  Basal  white  limestone. 

1.  White,  nodular,  cherty  limestone.  The  chert 
occurs  in  nodules  with  a  roughly  concentric 
structure  somewhat  suggestive  of  the  structure 

of  Cryptozoon _ _ _ —  1 '  6" 

2.  White,  cherty  limestone  carrying  the  chert  in 
thin  parallel  bands  which  are  etched  out  by  the 
weather  on  the  cross  sections.  The  surface  of 
each  chert  layer  shows  polygonal  cracks  sugges¬ 
tive  of  sun-cracks  in  shale.  This  structure 
belongs  to  each  separate  chert  layer  and  is  not 
a  columnar  structure.  The  weathered  surfaces 
of  these  chert  layers  are  dotted  with  small  cubic 
depressions  which  were  apparently  formed  by 
the  leaching  out  of  some  mineral  of  a  cubic 

habit _ _ — .  .  4'  6" 


Total . . . . . . . -  6' 


506  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 


Character 


Thickness 


b.  Argillaceous  and  calcareous  red  shale  and  limestone. 

1.  Soft,  purple  shale . . . . . . 

2.  Purple,  cherty  limestone . .. . 

3.  Purple  shale  with  occasional  bands  of  purple 

calcite . . . . . . 

4.  Purple,  crystalline  limestone _ _ _ 

5.  Alternating  layers  of  buff  and  chocolate-red 

shale  with  a  splintery  habit  of  weathering  and 
a  roughly  concretionary  structure.  Like  all  the 
succeeding  shales  and  sandstones  of  the  Unkar 
they  are  mottled  with  light-colored  spots  which 
are  usually  circular  or  elliptical  in  form  and  of 
all  sizes _ _ _ _ _ 

6.  Blue  limestone,  white  for  1"  at  the  base  and 

showing  dendritic  markings . . 

7.  Red  shale _ _ _ _ 

8.  Blue  limestone . . . 

9.  Red  shale  . . . . . . . . . 

10.  Purple  limestone  . . . . . . 

11.  Calcareous,  red  shale  with  three  thin  bands  of 

purple  limestone _ _ _ _ 

12.  Red,  crystalline  limestone  . . . . 

13.  Red  shale _ _ _ _ _ 

14.  Red,  crystalline  limestone . . . . 

1 5.  Red  shale . . . . . . . 

16.  Blue  limestone _ _ _ _ 

1 7.  Red  shale  . . . . . _  _ . 

18.  Blue  limestone _ _ 

19.  Pink  limestone  _ _ _ _ 

20.  Red  shale . . . . 

21.  Cherty,  white  limestone . . 

22.  Compact,  red  shale  forming  a  cliff.. . . 

23.  Alternating  layers  of  buff  and  red  shale . 

24.  Dense,  purple,  calcareous  shale  carrying  bands 

of  pink  calcite  and  forming  a  cliff.  Contains 
occasional  thin  bands  of  chert  . . 

25.  Blue,  calcareous  shale  with  an  onion-like  concre¬ 
tionary  structure  on  a  large  scale . . . 


4' 

1' 


5'  6" 

5" 
1'  5" 
4" 
3'  10" 

a  » 


9'  10" 
1' 

P 

1' 

11' 


3" 

1' 

2" 

1' 

4'  4" 
4" 


8' 

13'  6" 


9'  6" 
5' 


Total _ _ _ _  85'  5" 

c.  White  limestone. 

1.  Thin-bedded,  white,  cherty  limestone  carrying 

the  chert  in  parallel  bands,  and  containing  three 
paper-thin  layers  of  purple  shale.  Weathers 
to  a  white  powder.  Dendritic  markings  are 
present _ _ _ _ _  2' 

2.  Nodular,  white,  cherty  limestone.  The  chert  is 
present  in  irregular  nodules  of  no  definite  shape. 


The  Shinumo  Area. 


507 


Character  Thickness 

The  upper  part  of  the  stratum  has  a  paper-thin 
bedding  giving  it  the  aspect  of  a  calcareous  shale. 

The  limestone  weathers  to  a  white  powder. 

Dendritic  markings  are  present . . .  2' 

3.  Dense,  homogeneous,  white,  crystalline  lime¬ 

stone,  forming  a  cliff.  The  upper  part  is  thin- 
bedded  __ . . . . . . .  3'  8" 

4.  Homogeneous,  thin-bedded,  white,  crystalline 

limestone  containing  occasional  thin  bands  of 
chert  and  nodules  whose  character  suggest  the 
structure  of  Cryptozoon _  8'  6" 

5.  White  limestone  carrying  a  large  amount  of 

chert  in  undulatory  and  gnarled  bands _ ..  2'  4" 

6.  Lumpy  and  gnarly,  white  limestone  carrying 
chert  in  large  irregular  nodules.  Crumbles  to 

a  white  powder . . .  2'  6" 

V.  Purple  shale  . . . . . . 

8.  Layers  of  undulatory-banded,  bluish  chert _  1'  10" 

9.  Soft,  purple  shale  _ _  5" 

10.  Layer  of  gnarled  and  twisted  chert  nodules  in  a 
matrix  of  white  talc.  The  surface  of  the  talc 

is  covered  with  dendritic  markings  . .  2' 

1 1 .  Thin-bedded,  white  limestone  crumbling  to  white 

powder  or  weathering  into  thin  plates  like  a 
shale . 2' 

12.  Soft,  purple  shale . . . .  1' 

13.  Thin-bedded,  crystalline,  white  limestone .  3'  10" 

14.  Soft,  purple  shale . . . .  1'  10" 

15.  Dense,  blue,  crystalline  limestone  forming  a 

small  cliff . 3' 

16.  Homogeneous,  thin-bedded,  white  limestone, 
crumbling  to  a  white  powder  and  weathering 

into  plates  like  a  shale . 15' 

17.  Dense,  crystalline,  blue  limestone . .  4" 

18.  Purple  shale . . . . . .  3'  3" 

19.  Layers  of  undulatory-banded,  nodular  chert  in  a 

matrix  of  earthy  white  limestone .  3'  10" 

20.  Thick-bedded  layers  of  pure,  homogeneous, 
white  marble,  forming  the  strongest  cliff  in  the 

second  member  of  the  Unkar _ _  6'  8" 

21.  Same  in  thinner  beds _ _ _  6'  6" 

22.  Undulatory-banded,  cherty  limestone  becoming 

crystalline  above . . .  10' 

23.  Red  shale  below  and  purple  shale  above,  sepa¬ 
rated  by  a  thin  layer  of  chert . _ .  5' 

24.  Thick-bedded,  crystalline,  white  limestone  of 
the  same  character  as  20,  forming  a  strong 

cliff . . . . . .  3'  6" 


508  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 


Character  Thickness 

25.  Thin-bedded,  crystalline,  white  limestone _  5'  6" 

26.  Gnarled  and  nodular,  white,  cherty  limestone..  7' 

27.  Very  hard,  dense  layer  of  flint  forming  small 

cliff . . . - .  5" 


Total _ _ _ _ _ _ 

d.  Blue  slate  and  white  limestone. 

1.  Soft,  purple  shale . . . . . 

2.  Hard,  blue  slate  forming  small  cliff. . 

3.  Fissile,  blue  slates  with  fine  partings . . 

4.  Thin-bedded,  purple,  crystalline  limestone  _ 

5.  Dense,  purple,  crystalline  limestone  forming  a 

small  cliff _ _ _ _ _ _ 

6.  Same  as  4 _ _ _ _ _ 

7.  Same  as  5,  forming  small  cliff  . . . . 

8.  Thin-lamellar,  spotted,  blue  slate _ 

9.  Gnarly  layers  of  fine-lamellar,  blue  calcareous 

slate  with  a  very  coarse  concretionary  struc¬ 
ture.  Irregular  nodules  of  chert  occur  in  the 
middle  portion . . . . . . 

10.  Dense,  blue,  crystalline  limestone  forming  small 

cliff _ _ _ 

11.  Thin-bedded,  platy,  white  limestone . 

12.  Very  thin-lamellar,  fissile,  blue  slate _ _ 

13.  Calcareous,  blue  slate  forming  a  cliff _ _ 

14.  Thin-beddedT  platy  white  limestone _ 

15.  Fissile,  blue  slate _ _ 

16.  Dense,  lumpy,  white  crystalline  limestone  _ 

17.  Pinkish-green,  fissile,  siliceous  slate  of  a  jaspery 

appearance  forming  a  cliff  ..  . . . . . 

18.  Layers  of  dense,  white,  crystalline  limestone 

separated  by  thin  bands  of  pale-green,  talcose 
material _ _ _ _ 

19.  Dense,  red  and  black-banded  jasper,  weathering 

green  between  the  layers  and  forming  a  cliff. 
The  layers  contain  shrinkage  cracks  and  ripple 
marks  _ _ _ _ 

20.  Layers  of  dense,  white,  crystalline  limestone 

separated  by  bands  of  pale  green,  talcose 
material . . . . 

Total  _ _ _ _ _ 

Synopsis  of  second  member  of  the  TTnkar. 

a.  Basal  white  limestone _ _ _ _ 

b.  Argillaceous  and  calcareous  red  shale  and  limestone 

c.  White  limestone  . . . . 

d.  Blue  slate  and  white  limestone . . 


To 

o 

rH 

2" 

3' 

6" 

1' 

7' 

1' 

2" 

7" 

2' 

2" 

S" 

6' 

6" 

33' 

2' 

3" 

2' 

6" 

3' 

Q  f 

o 

4' 

6" 

7' 

2' 

5' 

10' 

11' 

4" 

2' 

108' 

2" 

6' 

85' 

5" 

105' 

2" 

108' 

2" 

Total  thickness 


304'  9" 


The  Shinumo  Area. 


509 


When  the  specimens  of  these  rocks  were  examined  in  the 
laboratory  it  was  found  that  all  the  limestones  were  more  or 
less  dolomitic.  The  limestones  of  division  “c”  were  found  to 
be  entirely  dolomites. 

Thin  sections  were  cut  from  specimens  from  twenty  separate 
beds  in  the  second  member.  Eighteen  of  these  slides  were  cut 
from  the  limestone  strata  and  two  from  the  red  shales.  The 
sections  of  the  limestones  were  cut  both  from  the  chert 
bands  and  the  nodules  and  from  the  limestone  itself,  for 
the  purpose  of  ascertaining  the  exact  mineralogical  char¬ 
acter  of  these  rocks,  and  in  the  forlorn  hope  that  they 
might  reveal  traces  of  a  structure  that  could  be  referred 
to  something  organic.  No  minerals  other  than  calcite  and 
quartz  were  revealed  by  the  microscope  in  any  of  the  slides. 
The  silica  of  the  chert  bands  and  nodules  was  found  to  exist 
in  the  form  of  interlocking  grains  of  quartz.  None  of  the 
grains  were  rounded  and  there  was  no  suggestion  that  the 
quartz  grains  of  the  chert  bands  represented  an  inwashed  sand. 
Nor  was  there  any  trace  of  an  organic  structure  revealed, 
either  in  the  chert  or  in  the  limestone.  The  purer  limestones 
were  found  to  consist  of  calcite  (or  dolomite)  alone,  the  crys¬ 
talline  forms  having  the  typical  structure  of  marble.  The 
impure  varieties  were  found  to  consist  of  mixtures  of  quartz 
and  calcite  in  all  proportions.  The  greater  part  of  the  lime¬ 
stone  wTas  of  this  impure  character.  The  shales  were  found  to 
consist  of  a  fine  impalpable  ferruginous  or  calcareous  mud, 
containing  occasionally  a  minute  grain  of  quartz. 

Several  features  of  interest  are  shown  in  the  lithologic  sec¬ 
tion  of  the  second  member  of  the  Unkar  as  a  whole. 

Nipple  marks  and  sun  cracks  appear  for  the  first  time  in  the 
shales  in  stratum  No.  18  of  division'  just  below  the  highest 
limestone  stratum  at  the  summit  of  the  member. 

The  increasing  intensity  of  metamorphic  phenomena  in  the 
section  from  the  base  upward  may  also  be  noted.  This  is  due 
to  the  approaching  proximity  to  the  lower  contact  of  the  dia¬ 
base  sill  which  is  intruded  in  the  member  above.  The 
metamorphic  action  is  manifested  in  the  shales  by  their  change 
upward  both  in  color  and  in  degree  of  induration :  the  shales 
of  division  “  b  ”  are  almost  entirely  red  ;  from  the  summit  of 
this  division  upward  the  color  changes  to  purple  and  blue. 
Below  division  ic  d”  the  shales  are  soft  and  crumbly;  within 
this  division,  however,  they  become  dark  blue  slates,  while  in 
the  upper  part  they  become  extremely  hard,  siliceous  jaspers. 

The  vertical  succession  of  the  strata  is  seen  to  be  broadly 
characterized  by  continual  and  rapid  alterations  of-  limestone 
and  shale.  According  to  the  dominance  of  either  type  of  rock 
the  four  divisions  a ,  b ,  <?,  aud  cl  are  separated  :  division  “  a  ”  is 


510  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

entirely  limestone,  ub ”  is  alternating  limestone,  and  u  d” 
predominantly  sliale.  Thus  there  are  four  major  cycles  of 
oscillation  upon  which  the  minor  cycles  are  superimposed. 

A  comparison  of  the  above  section  in  the  East  Wash  with  a 
section  measured  in  the  Asbestos  Canyon,  four  miles  to  the 
west,  is  of  interest.  In  the  Asbestos  Canyon  the  “  basal  white 
limestone  ”  (a)  has  a  thickness  of  30  feet  contrasted  with  a 
thickness  of  6  feet  in  the  East  Wash.  The  lower  stratum  of 
“nodular,  cherty  limestone  ”  is  there  7'  9",  contrasted  with 
V  3"  in  the  East  Wash.  The  upper  stratum  of  “parallel- 
banded,  cherty  limestone  ”  is  22'  3"  in  the  Asbestos  Canyon, 
containing  in  the  middle  an  intercalated  layer  of  purple  shale, 
and  near  the  top  a  thin  layer  of  rather  tine  arkose  conglomerate. 
The  “argillaceous  and  calcareous  red  shales  and  limestones  ”  of 
division  “  b have  a  thickness  of  88  feet  in  the  Asbestos 
Canyon  contrasted  with  85'  5"  in  the  East  Wash.  They  do 
not  have  the  red  color  that  characterizes  them  in  the  East  Wash, 
but  are  purple  and  blue,  and  much  indurated.  This  change  of 
color  and  difference  in  degree  of  induration  is  due  to  their 
closer  proximity  to  the  diabase  sill  in  the  Asbestos  Canyon. 

The  correspondence  in  lithographical  character  and  vertical 
succession  of  these  two  sections  four  miles  apart  is  so  close  that 
the  individual  strata  of  the  sections  can  be  matched  bed  for 
bed.  The  only  marked  contrast  in  thickness  occurs  in  the 
basal  white  limestone  (a). 

Third  Member. — Argillaceous  and  Arenaceous  Shale. 

Section  measured  at  East  Wash  in  continuation  of  the  pre¬ 
ceding  section. 

a.  Cliff -forming  gasper. 

1.  Dense,  hard  layer  of  blue-black  jasper  mottled 
with  red  spots  and  showing  no  banding  in  the 
mass,  forming  with  the  three  following  layers  a 
strong  perpendicular  cliff.  This  is  the  most 
resistant  rock  in  the  Unkar.  Where  the  under 
surface  shows  beneath  the  overhang  of  the  cliff 
it  is  sun-cracked  on  a  large  scale  and  in  several 


generations. _ _ _ _  28' 

2.  Same  general  character  as  1,  but  showing  a 

banded  structure.  The  lower  2  ft.  are  slaty  and 
weather  out,  giving  the  cliff  an  overhang _  19' 

3.  Same  as  2  with  a  soft  slaty  layer  at  the  base...  14' 

4.  Same  as  2  and  3  with  a  soft  layer  at  the  base.  12' 

Total  73' 


The  Shinumo  Area. 


511 


b.  Calcareous  blue  slate. 

1.  Slaty,  black  jasper,  sun-cracked  throughout...  12' 

2.  Pink,  crystalline  limestone . p  q" 

3.  Slaty,  blue  jasper  with  small  red  spots .  4'  6" 

Total  18' 

c.  Cliff -forming  jasper. 

1.  Dense,  hard  layer  of  blue-black  jasper  mottled 

with  red  spots,  forming  with  the  following 
layer  a  strong  cliff _ _  14' 

2.  Same  as  1  with  a  soft  slaty  layer  at  the  base 

which  weathers  out,  giving  the  cliff  an  overhang.  3'  6" 

Total  17'  6" 

cl.  Sanclg  quartzitic  jasper. 

1.  Slaty,  blue,  spotted  jasper . .  4' 

2.  Fine-grained,  pink  quartzite,  ripple-marked _  5' 

3.  Slaty,  blue,  spotted  jasper.. . . .  12' 

*  4.  Fine-grained,  pink  quartzite _ 4' 

5.  Slaty,  blue,  spotted  jasper  with  sun-cracks .  6' 

6.  Fine-grained,  pink  quartzite,  ripple-marked...  l' 

7.  Pink,  quartzitic  jasper . 5' 

8.  Fine-grained,  pink  quartzite... .  . 4' 

9.  Fine-grained,  pink,  sandy  jasper,  sun-cracked. .  1 1' 


Total  52' 

e.  Red  and  blue  jasper . 

1.  Banded,  blue  jasper  with  curious  spots.  Sun- 

cracked  throughout  . . 22' 

2.  Red  and  black  banded  jasper _ _ 9' 


Total  31' 

f.  At  this  horizon  is  intruded  a  sill  of  diabase,  whose 
thickness  varies  from  650  feet  on  the  Shinumo 
to  950  feet  or  more  in  the  Asbestos  Canyon. 


The  remainder  of  the  section  was  measured  in  a  traverse  up 
the  Shinumo,  starting  with  the  upper  contact  of  the  diabase 


sill. 

g.  Blue  slate  and  quartzite  forming  a  cliff .  20' 

h.  Blue  slate  forming  a  slope _ _ _ _  100' 

i.  Red,  argillaceous  shale  —  sun-cracked  throughout. 

The  rock  is  very  soft  and  forms  a  slope  together 
•  with  the  underlying  blue  slate.. . .  91' 

j.  Alternating,  vermilion,  argillaceous  shale  and  sand¬ 

stone. 


The  alternations  in  this  series  occur  with  remarkable  reg¬ 
ularity.  The  sandstone  is  white  in  color  and  is  compact  and 


512  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 


fine-grained.  It  is  cross-bedded  and  ripple-marked  through¬ 
out.  The  shales  of  the  alternating  beds  are  very  soft  and 
weather  out,  leaving  etched-out  bands  between  the  sandstones 
which  are  very  conspicuous  in  the  cliff-faces.  On  the  under¬ 
surface  of  each  sandstone  layer  are  beautifully  preserved  sun- 
cracks.  The  shales  are  tine-grained,  fissile,  and  argillaceous. 

The  succession  in  this  alternating  series  is  as  follows : 


No. 

Bed 

Thickness 

S.  S.— Sh.  cycle 

1. 

Sandstone  _ 

4'  2" 

2. 

Shale.  _ _ 

...  3 To"  ... 

. .  8' 

3. 

S.  S _ 

...2' 

4. 

Sh _ 

...  3' 

_  5' 

5. 

s.  s _ 

...  1' 

6. 

Sh _ 

...2' 

_  3' 

7. 

s.  s _ 

...  V' 

8. 

Sh _ 

.  .  .  10'  6"  ... 

_ 17'  6" 

9. 

s.  s _ 

...  5' 

10. 

Sh _ 

...3' 

. .  8' 

11. 

s.  s _ 

...  6' 

12. 

Sh _ 

...  2'  6"  ... 

.  . . .  8'  6" 

13. 

s.  s _ 

...  6' 

14. 

Sh _ 

...  3'  6"  ... 

. .  9'  6" 

15. 

s.  s _ 

...  6' 

16. 

Sh _ 

..  .  3'  6"  ... 

_  9'  6" 

IV. 

s.  s _ 

...  V'  6" 

18. 

Sh _ 

...  4' 

_ 11'  6" 

19. 

s.  s _ 

.  . .  3'  6" 

20. 

Sh . . 

6"  .... 

_ _  4' 

21. 

s.  s _ 

.  .  .  2' 

22. 

Sh _ 

...  r  _ 

. . .  3' 

23. 

s.  s _ 

. ..  3 To" 

24. 

Sh _ 

...  2'  6"  ... 

_ _  6'  4" 

25. 

s.  s _ 

2' 

26. 

Sh _ 

...  6'  6"  ... 

.  . .  8'  6" 

2V. 

s.  s _ 

...  3' 

28. 

Sh _ 

...  4' 

. .  7' 

Total, 

109'  4" 

Ave.,  7  TO" 

Jc.  Alternating,  vermilion,  arenaceous  shale  and  sandstone. 

The  sandstone  is  white,  compact,  and  fine-grained.  It  is 
cross-bedded  and  ripple-marked  throughout.  The  under¬ 
surface  of  each  sandstone  layer  is  sun-cracked  where  it  rests 
upon  the  arenaceous  shale.  The  shale  is  vermilion  in  color, 
soft,  and  very  sandy.  Sun-cracks  occur  throughout. 

The  succession  is  as  follows : 


The  Shinumo  Area. 


513 


1.  Sandstone . 9'  4 

2.  Arenaceous  shale .  21'  1" 

3.  Sandstone . 9' 

4.  Arenaceous  shale . . . 11'  8" 

5.  Sandstone . . . 2' 

6.  Arenaceous  shale . . .  24' 

7.  Sandstone . 1' 


Total  78'  1" 

Synopsis  of  third  member  of  the  Unkar. 

a.  Cliff-forming  jasper . .  73' 

b.  Blue  slate  with  calcareous  band _  18' 

c.  Cliff-forming  jasper . . 17'  6" 

d.  Sandy  quartzitic  jasper . . 52' 

e.  Red  and  blue  jasper_ . .  31' 

f  (Intrusive  diabase) 

g.  Blue  slate  and  quartzite... _  20' 

h.  Blue  slate . . 100' 

i.  Red  argillaceous  shale . .  81' 

j.  Alternating  vermilion  argillaceous 

shale  and  S.  S . . 109'  4" 

k.  Alternating  vermilion  arenaceous 

shale  and  S.  S . . .  78'  l" 


Total  thickness  5 7 9' 11" 

Thin  sections  were  cut  from  several  specimens  of  the 
jaspers.  The  slides  were  unsatisfactory,  however,  because  of 
the  exceedingly  fine  grain  of  the  rock.  The  highest  power  of 
the  microscope  revealed  nothing  more  than  an  impalpable 
silicified  mud.  A  slide  of  the  “  quartzitic  jasper  ”  showed  it 
to  have  been  a  somewhat  arkose  sandstone  indurated  to  a 
siliceous  quartzite.  It  was  seen  to  be  composed  chiefly  of 
small  rounded  quartz  grains  about  which  secondary  silica  had 
been  deposited,  lying  in  a  fine  arkose  matrix  made  up  of  small 
fragments  of  pink  feldspar.  A  thin  section  wras  also  made 
from  a  specimen  of  one  of  the  sandstone  layers  in  the  “  alter¬ 
nating  argillaceous  shale  and  sandstone”  of  division  uj .  ” 
The  rock  proved  to  consist  of  small,  well-rounded  grains  of 
quartz,  cemented  by  silica  in  the  form  of  secondary  quartz.  It 
is  a  pure,  fine-grained  sandstone. 

The  metamorphic  effects  produced  by  the  diabase  sill 
intruded  at  the  horizon  uf”  are  seen  in  the  lithologic  section  of 
the  third  member  given  above.  This  metamorphic  action  is 
manifested  in  three  ways  : 

1.  Induration  by  silicification, — jaspers. 

2.  Induration  by  baking, — slates. 

3.  Decoloration, — red  to  blue  and  black. 

Am.  Jour.  Sci. — Fourth  Series,  Vol.  XXIX,  No.  174. — June,  1910. 

34 


514:  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

It  was  noted  in  the  summary  of  the  features  of  the  second 
member  that  the  shales  became  successively  slates  and  jaspers 
above,  while  their  color  changed  from  red  to  blue.  In  the 
third  member  the  shales  are  represented  entirely  by  jaspers 
and  quartzites.  Just  below  the  contact  the  induration  is  very 
great,  and  the  jaspers  are  tough  and  vitreous ;  the  prevailing 
color  is  blue  or  black.  Above  the  contact  the  overlying  rocks 
are  hard  blue  slates  for  20  feet,  succeeded  by  100  feet  of  less 
indurated  slate,  grading  upward  into  the  original  red  shale. 

The  metamorphic  effects  above  and  below  the  contact  differ 
in  degree  of  intensity  as  well  as  in  kind :  above  the  contact 
the  induration  and  decoloration  characterize  only  about  100 
feet  of  strata ;  below  the  contact  this  action  extends  through 
300  feet.  Above  the  contact  the  strata  are  characterized  by 
baking  and  decoloration  only,  changing  the  red  shale  into  a 
blue  slate  ;  below  the  contact  there  has  been  a  considerable 
addition  of  silica,  transforming  the  red  shales  into  blue  and 
black  jaspers ;  added  to  this  are  the  effects  of  baking  and 
decoloration. 

It  may  be  said  in  summary  that  the  third  member  is  charac¬ 
terized  by  argillaceous  shales  in  the  lower  portion,  which  grade 
upward  into  arenaceous  shales  and  sandstones  through  the 
interesting  series  of  alternations  described  in  division  uj.” 
There  is  hardly  a  stratum  in  the  entire  member  that  does  not 
bear  marks  of  shallow  water  origin,  manifested  by  either  sun- 
cracks,  ripple-marks,  or  cross-bedding. 

Fourth  Member. — Sandstone  and  Quartzite. 

Section  on  the  Shinumo. 

a.  Purple-brown  sandstone  of  fine  grain  containing  locally. 

an  occasional  lense  of  fine  conglomerate  and  sometimes 
a  thin  local  bed  of  red  or  purple  shale.  The  sand¬ 
stone  is  cross-bedded  throughout .  406' 

b.  Compact,  white  quartzite  of  fine  and  uniform  grain, 

displaying  a  faint  cross-bedded  structure.  This  quartz¬ 
ite  is  the  most  resistant  rock  in  the  fourth  member.  It 
is  exposed  everywhere  in  one  massive  perpendicular 
cliff  face,  which  does  not  display  the  slightest  break 
except  where  it  is  cut  by  faults.  Wherever  its  base 
rests  upon  a  shaly  lense  the  under  surface  displays 
well  preserved  mud-cracks.  The  face  of  the  cliff  is 
stained  magenta  by  the  ferruginous  cement  of  the 
shale  lenses  in  the  overlying  sandstones,  which  washes 


down  from  above . . . . . .  119' 

c.  Purple-brown  sandstone  of  the  same  character  as  a _  353' 

d.  Banded,  white  quartzite,  stained  magenta  on  the  expo¬ 

sures,  and  forming  a  cliff . . .  120' 


The  Sfiinumo  Area. 


515 


e.  Fine-grained,  purple  sandstone  with  a  white  band  in  the 

middle.  The  white  band  is  constant  and  presents  a 
conspicuous  feature  by  which  this  purple  sandstone 
can  be  distinguished  at  a  distance  of  several  miles. 

The  rock  is  cross-bedded  and  sometimes  displays  a 
“  twisted  and  gnarled  ”  structure . __  150' 

f.  Compact,  cliff-forming,  white  quartzite  of  the  same 

character  as  b ,  though  not  so  massive  in  structure _  250' 

g.  Banded  white  quartzite _ _ _ _  20' 

h.  Irregularly  bedded  sandstone  ....  . . . 


1.  “  Curiously  twisted  and  gnarled  layers,”  of  fine¬ 
grained  white  sandstone  containing  large  red  spots  of 
a  circular  and  elliptical  form.  The  upper  part  of  the 
bed  is  more  massive.  The  twisted  and  gnarled  struc¬ 
ture  seems  to  have  been  a  phenomenon  of  the  original 
deposition.  It  gives  the  rock  an  appearance  which 
suggests  that  the  original  sand  was  moist  and  plastic 
and  once  flowed  by  rolling  over  and  over  in  the  form 
of  a  quicksand.  105' 

(It  may  be  noted  that  a  bed  of  this  character  is 
described  by  Walcott  in  his  section  in  Unkar  Val¬ 
ley.  (Walcott  5,  page  511.)  It  occurs  at  the  same 
horizon  as  the  bed  described  above,  and  contains  the 


same  red  spots.) 

2.  Banded,  purple  sandstones . .  20' 

3.  Green,  cross-bedded  sandstone  . . .  21' 


146' 

The  total  thickness  of  the  fourth  member  of  the  Unkar  is 
1564  feet. 

Slides  were  cut  from  several  specimens  of  the  sandstones 
and  quartzites.  All  were  found  to  consist  of  small  rounded 
quartz  grains,  the  size  of  which  seldom  exceeds  0*7mm.  This 
extreme  fineness  and  roundness  of  the  grains,  as  well  as  the 
cleanness  of  the  sorting,  is  remarkable.  The  cement  is  usually 
siliceous,  sometimes  slightly  ferruginous.  A  slide  was  made 
from  a  specimen  taken  from  one  of  the  small  conglomerate 
lenses  in  division  a.  The  conglomerate  was  found  to  consist 
of  small  rounded  quartz  pebbles,  lying  in  a  fine  arkose  matrix. 
Occasional  large  angular  fragments  of  ortlioclase  and  micro- 
cline  were  revealed. 

In  summary  it  may  be  said  that  the  fourth  member  is  char¬ 
acterized  by  great  thicknesses  of  pure,  fine-grained  sandstone 
of  a  uniform  character.  All  divisions  of  the  member  are 
resistant  to  the  weather  and  form  cliffs.  Bipple  marks  and 
cross-bedding  occur  throughout. 

Fifth  Member. — Micaceous  Shaly  Sandstone. 

Section  measured  in  Fault  Wash,  west  side  of  Shinumo 
Canyon. 


516  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

a.  Gray-green,  pinkish-green,  and  brown  micaceous  shaly 

sandstones,  cross-bedded  and  ripple-marked,  varying 
in  character  only  through  gradations  in  color.  Fre¬ 
quent  arenaceous  and  argillaceous  shaly  partings  occur, 
causing  the  rock  to  weather  like  a  soft  sandy  shale. 

The  shale  partings  are  usually  green.  Some  of  the 
sandy  layers  near  the  base  show  the  “gnarled  and 
twisted  structure.”  . . . . . .  1100' 

b.  The  green,  gray,  and  brown  beds  of  a  pass  upward 

through  change  of  color  into  red  and  vermilion  beds 
of  the  same  general  character.  These  are  likewise 
micaceous  shaly  sandstones,  cross-bedded  and  ripple- 
marked,  with  arenaceous  and  argillaceous  shaly  part¬ 
ings,  which  display  well  preserved  mud-cracks.  The 


shaly  partings  are  either  green  or  red . . .  1197' 

Total . . .  2297' 


The  summit  of  the  Unkar  section  is  obliterated  in  the  beds 
of  division  b  by  the  plane  of  the  pre-Tonto  unconformity. 
The  highest  beds  of  this  division  lie  at  the  upper  end  of  the 
Fault  Wash,  dragged  against  the  Vishnu  schist  on  the  north 
by  the  great  pre-Cambrian  fault  previously  described,  and  the 
whole  overlain  by  the  Tonto  sandstone. 

In  the  uppermost  part  of  division  b  are  four  thin  sills  of 
rotten  basaltic  rock,  weathering  green  and  crumbling  to  small 
fragments.  They  occur  between  the  beds  of  vermilion  sand¬ 
stone  and  shale.  Their  intrusive  character  is  shown  by  the 
fact  that  they  have  baked  and  decolorized  the  vermilion 
beds  for  a  few  inches  both  above  and  below  the  contacts, 
changing  the  color  to  purple.  The  rock  is  too  badly  weathered 
to  allow  a  petrographic  determination  in  thin  section. 

In  summary  the  fifth  member  of  the  Unkar  may  be  charac¬ 
terized  as  a  series  of  micaceous  shaly  sandstones  of  uniform 
character,  varying  only  in  color,  and  bearing  marks  of  shallow 
wrater  origin  throughout. 

Comparison  with  the  type  section  in  TJnkar  Valley. — 
A  comparison  of  the  above  section  of  the  Unkar  group  with 
that  described  by  Walcott  (d)  in  the  type  locality  30  miles  to 
the  east  reveals  the  fact  that  the  correspondence  between  the 
two  sections  is  very  close,  both  in  thickness  and  in  lithological 
succession  ;  only  in  the  lower  portions  do  they  differ  materi¬ 
ally.  The  type  section  in  Unkar  valley  is  characterized  by  a 
greater  thickness  of  the  basal  conglomerate,  and  by  only  a  third 
as  much  limestone  in  the  members  which  correspond  to  the  sec¬ 
ond  member  on  the  Shinumo ;  the  deficiency  in  limestone  is  made 
up  by  greater  thickness  of  arenaceous  and  argillaceous  shale. 
Proceeding  upward  in  the  section  there  is  a  somewhat  greater 


The  Shinumo  Area. 


517 


proportion  of  sand  in  that  division  of  Walcott’s  section  which 
corresponds  to  the  third  member  on  the  Shinumo.  The  succeed¬ 
ing  members  correspond  closely  in  character  and  thickness 
even  to  the  minor  divisions  ;  an  example  of  this  is  the 
“gnarled  and  twisted  layers”  previously  cited. 

The  writer  had  the  privilege  of  examining  Mr.  Walcott’s 
field  specimens  in  the  National  Museum  in  Washington  and 
was  particularly  impressed  by  their  absolute  lithological  iden¬ 
tity  with  the  series  collected  by  himself  from  corresponding 
horizons  on  the  Shinumo.  The  two  series  of  specimens,  with 
the  exception  of  those  rocks  altered  by  local  metamorpliic  phe¬ 
nomena,  might  have  come  from  the  same  locality. 

Diabase  Intrusive  in  the  Unkar. 

Occurrence. — The  diabase  occurs  in  the  form  of  an  intrusive 
sheet  or  sill  which  occupies  three  separate  stratigraphic  hori¬ 
zons  in  the  TJukar  sediments  in  different  parts  of  the  Shinumo 
area.  On  the  west  side  of  the  Asbestos  Canyon  three  miles 
west  of  the  Shinumo  it  is  intrusive  in  the  limestones  of  division 
c  of  the  second  member  of  the  Unkar  at  a  horizon  15  feet  above 
their  base.  On  the  east  side  of  the  Asbestos  Canvon  it  ascends 
out  of  this  horizon  and  breaks  across  the  overlying  limestones 
and  shales  in  an  eruptive  contact.  Most  of  the  lower,  and  part 
of  the  upper,  sections  of  the  eruptive  contact  are  beautifully 
displayed  in  the  walls  of  this  canyon,  but  the  complete  expo¬ 
sure  is  obliterated  above  by  the  truncating  unconformity 
at  the  base  of  the  Tonto  sandstone.  Between  the  exposures  in 
the  Asbestos  Canyon  and  the  main  areal  exposures  about  the 
mouth  of  the  Shinumo,  the  pre-Cambrian  structure  is  hidden 
beneath  the  sandstone  of  the  Tonto  platform.  Where  the  dia¬ 
base  reappears  in  the  exposures  about  the  Shinumo  it  is  found 

to  lie  intruded  midwav  within  the  shales  of  the  third  mem- 

«/ 

her  of  the  Unkar  at  a  horizon  400  feet  above  that  which 
it  occupies  in  the  Asbestos  Canyon.  This  is  the  stratigraphic 
position  which  it  holds  in  all  the  exposures  of  the  central  part 
of  the  Shinumo  area.  Eastward  up  the  river  from  the  East 
Wash  the  diabase  again  disappears  beneath  the  pre-Tonto  un¬ 
conformity.  But  directly  under  Iiavasupai  Point,  about  three 
miles  farther  east,  limited  outcrops  of  the  basal  portion  are 
exposed  in  small  inter-canyon  valleys  on  both  sides  of  the  river. 
Here  the  lower  contact  of  the  diabase  lies  just  at  the  top  of 
the  u  white  limestones”  of  division  c  of  the  second  member 
of  the  Unkar. 

The  thickness  of  the  sill  is  approximately  650  feet  on  the  Shi¬ 
numo  and  950  feet  on  the  west  side  of  the  Asbestos  Canyon. 
Eastward  from  the  Shinumo  the  outcrop  is  considerably 


518  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

thinned  by  a  strike  fault,  precluding  the  possibility  of  measur¬ 
ing  the  thickness  in  that  direction. 

ij 

Petrography. 

Megascopic. — The  fresh  specimens  characteristic  of  the 
greater  part  of  the  mass  show  it  to  be  a  tough,  heavy,  holo- 
crystalline  rock  of  medium  to  coarse  grain  and  of  a  grey  color. 
The  minerals  visible  to  the  unaided  eye  are  plagioclase,  olivine, 
augite,  and  an  occasional  grain  of  magnetite.  Although  the 
olivine  exceeds  the  augite  in  amount,  it  is  less  conspicuous  to 
the  eye.  Aside  from  a  somewhat  waxy  luster  of  the  feldspars, 
the  rock  is  remarkably  fresh.  The  weathered  surface  has  a 
characteristic  warty  appearance,  imparted  by  the  presence  of 
lumps  or  balls  which  are  of  a  coarser  grain  and  different  tex¬ 
ture  than  that  of  the  mass  of  the  rock  and  are  more  resistant 
to  the  processes  of  disintegration.  These  lumps  and  balls  can 
be  seen  to  consist  of  coarse  ophitic  intergrowths  of  augite  and 
plagioclase.  The  diabase  weathers  by  mechanical  disintegra¬ 
tion  to  a  greenish-olive  sand  in  which  are  innumerable  lumpy 
kernels  of  all  sizes  derived  from  the  ophitic  masses  described 
above.  The  rock  has  no  typical  columnar  structure,  but  usu¬ 
ally  displays  a  rough  vertical  jointing  such  as  is  characteristic 
of  granite. 

Microscopic. — The  slides  examined  show  the  typical  rock  to 
consist  primarily  of  plagioclase  feldspar  (near  labradorite)  and 
olivine  in  about  equal  amounts,  with  a  subordinate  quantity  of 
augite  and  brown  biotite.  A  very  little  magnetite  is  present. 
The  feldspar  is  somewhat  altered,  but  all  the  other  minerals 
are  fresh.  The  olivine  occurs  characteristically  in  rather  large, 
rounded  crystals  of  automorphic  habit.  The  augite  is  chiefly 
confined  to  the  globular  masses  which  weather  out  as  lumps 
and  kernels  and  does  not  characterize  the  rock  as  a  whole. 
Slides  cut  from  these  kernels  show  them  to  be  conqDosed 
entirely  of  augite  and  feldspar.  The  augite  is  enclosed  within 
the  feldspar,  displaying  beautiful  examples  of  ophitic  texture. 
Several  of  the  magnetite  crystals  were  observed  to  have  rims 
of  brown  biotite.  The  small  amount  of  magnetite  is  rather 
remarkable,  and  it  seems  likely  on  this  account  that  the  olivine 
is  rich  in  magnesia.  Because  of  the  predominance  of  olivine 
and  plagioclase  in  the  greater  part  of  the  rock,  the  diabase  is 
classified  as  an  olivine-diabase  with  a  troctolitic  aspect. 

Variations  in  character. — All  parts  of  the  mass  are  subject 
to  variations  in  texture  and  composition  toward  a  coarser  grain. 
These  are  of  two  kinds.  The  first  type  occurs  in  the  ophitic 
intergrowths  of  augite  and  plagioclase  in  the  lumps  and  balls 
described  above,  and  is  a  segregation  phenomenon  character- 


The  Shinumo  Area. 


519 


izing  the  mass  as  a  whole ;  sometimes  this  texture  becomes 
very  coarse,  the  separate  crystals  of  augite  or  plagioclase  attain¬ 
ing  an  inch  in  length.  The  second  type  occurs  in  typical  peg¬ 
matite  dikes  cutting  the  diabase  in  many  places  and  varying  in 
width  from  a  few  inches  to  several  feet.  -  The  minerals  are 
plagioclase  and  augite  and  the  texture  is  usually  ophitic, 
although  not  always  so.  In  some  of  these  dikes  crystals  of 
plagioclase  exceeding  three  inches  in  length  were  observed. 

The  contact  facies  of  the  diabase  are  frequently  line-grained 
or  glassy,  but  never  for  more  than  a  few  inches  from  the  con¬ 
tact.  The  slides  typical  of  this  zone  reveal  a  hyalopilitic 
arrangement  of  glass  with  skeleton  crystals  of  magnetite 
between  badly  altered  crystals  of  feldspar. 

For  about  a  half-mile  east  and  west  of  the  Shinumo  there 
occurs  in  the  upper  part  of  the  diabase  sill  along  the  upper 
contact  a  pink  holocrystalline  rock  of  medium  grain.  The  con¬ 
tact  of  this  rock  with  the  overlying  blue  slates  is  sharp  and 
well-defined.  Downward  it  appears  to  grade  into  the  normal 
diabase,  and  no  definite  line  of  contact  can  anywhere  be 
observed.  Unfortunately  the  writer  did  not  collect  transition 
specimens,  but  took  only  one  specimen  from  the  middle  of  the 
pink  mass.  The  slide  from  this  specimen  when  examined 
under  the  microscope  showed  it  to  be  a  granular  rock  of 
medium  texture,  consisting  of  rather  fresh  crystals  of  ortho- 
clase,  with  subordinate  quartz  and  a  somewhat  altered  ferro- 
magnesian  mineral  which  was  made  out  to  have  been  originally 
a  hornblende.  Some  of  the  quartz  displayed  a  micrographic 
arrangement  within  the  feldspar.  The  rock  is  a  typical  horn- 
blende-syenite,  and  is  apparently  an  interesting  example  of 
differentiation  in  place  within  the  diabase  sill.  But  a  more 
complete  set  of  specimens  across  the  apparent  transition  zone 
must  be  collected  before  such  a  conclusion  can  definitely  be 
established. 

In  the  Asbestos  Canyon  both  the  lower  and  upper  eruptive 
contacts  of  the  diabase  are  ragged  and  considerably  injected. 
Many  small  dikes  penetrate  the  country  rock  from  the  main 
mass.  They  are  glassy  in  texture  and  badly  altered. 

Bansome,  in  his  report  upon  the  u  Geology  of  the  Globe 
Copper  District,  Arizona  ”  (Bansome  b ,  p.  80  ff.)  describes  a 
diabase  of  post-Carboniferous  age  occurring  in  thick  sills  in 
the  pre-Carboniferous  sedimentary  rocks  of  that  region.  This 
diabase  closely  resembles  the  Algonkian  diabase  described 
above,  both  in  mineralogical  character  and  in  the  presence  of 
the  ophitic  balls  of  plagioclase  and  augite.  The  analogy  is 
made  the  more  striking  by  the  fact  that  several  small  masses 
of  pink  hornblende-syenite  are  described  occurring  within 
the  diabase  sills  of  the  Globe  district  as  a  possible  segregation 
phenomenon  within  the  diabasic  magma. 


520  Noble — Geology  of  the  Grand  Canyon,  Arizona. 

Contact  metamorphism. — Since  the  diabase  sill  occupies 
relatively  different  horizons  in  the  Unkar  strata  upon  the 
Shinmno  and  in  the  Asbestos  Canyon,  and  since  in  the  former 
locality  the  strata  between  which  the  sill  is  intruded  lie  in 
undisturbed  sedimentary  contact  in  the  latter  locality,  and 
vice  versa,  the  study  of  the  respective  contact  effects  upon  the 
invaded  strata  could  be  made  with  great  facility. 

The  contact  effect  upon  the  shales  which  lie  above  and 
below  the  diabase  along  the  Shinumo  has  already  been 
described  in  the  detailed  section  of  the  Unkar:  there  the 
shales  were  shown  to  be  altered  to  jaspers  by  baking  and  silici- 
fication.  It  was  shown  that  the  intensity  of  metamorphic 
action  was  much  greater  below  the  sill  than  above,  extending 
through  300  feet  of  strata  below  the  lower  contact  and  through 
only  100  feet  above  the  upper  contact.  In  the  Asbestos 
Canyon  these  rocks  lie  in  undisturbed  sedimentary  contact 
and  are  there  unaltered  red  shales. 

The  contact  effect  upon  the  limestones  can  be  studied  in  the 
Asbestos  Canyon,  where  the  diabase  sill  lies  intruded  within 
them.  Immediately  below  the  lower  contact  of  the  diabase, 
which  is  sharp  and  well  defined,  is  a  thin  layer  of  green  ser¬ 
pentine.  Below  lie  layers  of  pure  crystalline  limestone  (dolo¬ 
mite)  alternating  with  layers  of  the  same  character  containing 
bands  and  nodules  of  serpentine.  Within  one  of  the  layers 
containing  the  bands  and  nodules  of  serpentine  are  cross¬ 
fiber  veins  of  golden-yellow  chrysotile  asbestos  which  are 
parallel  in  general  trend  to  the  bedding  of  the  limestone. 
These  limestones  are  the  layers  at  the  base  of  division  c  of  the 

second  member  of  the  Unkar.  They  overlie  the  red  shales  of 

•/ 

division  b ,  which  are  here  baked  to  blue  slates. 

Section  below  diabase  contact  (reading  upward  from  the  base) 


Dense,  purple,  calcareous  slate  (5,  24  of  second  member 

of  Unkar) . 9' 

Soft,  blue  slate  ( b ,  25) _ _ _ _ _ _  3' 

Nodular,  cherty  limestone  (c,  1,  2)  . . . .  4' 

Banded,  crystalline  limestone  with  bands  and  nodules  of 

serpentine  (c,  3,  4,)  .  .  _ _ _  10' 

Serpentinous,  nodular  and  banded  layer  carrying  veins 

of  asbestos . T 

White,  crystalline  limestone  with  bands  and  nodules  of 

serpentine . . . 2' 

Pure,  white,  crystalline  limestone . .  ]'  6" 

Layer  of  green  serpentine _ 2' 

Diabase. 


Above  the  upper  contact  of  the  diabase  the  limestones  con¬ 
tain  several  alternating  layers  of  green  serpentine  and  narrow 


The  Shinumo  Area. 


521 


veins  of  asbestos  occur  at  several  horizons  in  proximity  to  the 
contact. 

The  geological  occurrence  of  the  asbestos  is  fully  described 
by  Diller  in  “  Mineral  Resources  of  the  U.  S.  for  1907”  (Diller, 
a\  and  again  by  the  same  writer  in  “  Mineral  Resources  for 
1908  ”  (Diller,  h).  The  contribution  of  the  writer  of  the 
present  article  consists  of  the  further  data  afforded  by  a  micro¬ 
scopic  study  of  the  rocks  associated  with  the  asbestos  in  this 
locality.  A  microscopic  study  was  made  of  25  thin  sections 
cut  from  the  limestones,  the  bands  and  nodules  of  serpentine, 
and  across  the  veins  of  asbestos.  Aside  from  the  serpentine 
and  asbestos  no  other  minerals  were  revealed  in  the  limestones 
beyond  the  dolomitic  calcite  and  interlocking  grains  of  quartz 
already  described  in  the  slides  cut  from  the  limestones  of  the 
same  horizon  in  the  section  on  the  East  Wash,  where  the  same 
strata  lie  in  undisturbed  sedimentarv  contact.  The  texture  of 
the  limestones  is  entirely  that  of  marble.  The  serpentine  of 
the  bands  and  nodules  shows  no  trace  of  an  alteration  structure 
which  might  indicate  a  derivation  from  pyroxene,  hornblende, 
or  olivine.  The  slides  cut  across  the  veins  of  asbestos  showed 
them  to  be  later  than  the  serpentine  in  which  they  are  usually 
enclosed.  A  great  number  of  veins  of  asbestos  of  microscopic 
size  was  revealed  in  some  of  the  slides  where  their  presence 
was  unsuspected.  Some  of  these  veins  were  observed  cutting 
across  both  the  serpentine  and  the  limestone  in  the  same  slide. 

The  asbestos  which  occurs  in  the  larger  veins  is  commercially 
of  high  grade  and  the  liber  is  of  great  tensile  strength.  It  is 
pronounced  by  Diller  to  be  the  best  in  quality  yet  found  any¬ 
where  in  the  United  States.  (Diller,  5,  p.  11.)  Locally  the 
crossfiber  is  4  inches  in  length.  The  horizon  of  the  larger 
veins  is  confined,  so  far  as  is  known,  to  the  limestones  which 
lie  beneath  the  lower  contact  of  the  diabase  sill.  The  veins 
above  the  contact,  although  more  widely  distributed  through 
the  limestones,  are  usually  of  smaller  size.  The  horizon  below 
the  contact  is  not  absolutely  constant  in  stratigraphic  position 
and  may  lie  anywhere  from  3  to  15  feet  below  the  contact. 
The  width  of  the  veins  within  this  horizon  varies  greatly  from 
place  to  place,  so  that  a  vein  of  three  inches  in  width  in  one 
locality  may  be  represented  by  a  zone  of  innumerable  small 
veins  in  another,  but  the  actual  continuity  of  the  zone  that 
carries  the  asbestos  is  rarely  broken. 

Conclusions. — The  serpentine  and  asbestos  occur  in  the 
limestones  only  where  these  strata  are  invaded  bv  the  dia¬ 
base  sill ;  where  the  diabase  lies  between  shales  there  is  no 
development  of  these  minerals  within  the  invaded  strata.  In 
no  place  in  the  area  are  they  developed  within  the  diabase 
itself.  It  is  therefore  clear  that  they  are  a  contact  metamorphic 


522  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

phenomenon  conditioned  by  the  invasion  of  the  limestones  by 
the  diabase.  It  seens  probable,  as  suggested  by  Diller  (a,  p. 
72),  that  the  serpentine  which  encloses  the  veins  of  asbestos  is 
derived  from  some  mineral  in  the  limestones  and  not  from  the 
diabase.  The  limestones  themselves  are  magnesian,  and  locally 
siliceous  in  the  form  of  chert  bands  and  nodules.  In  another 
part  of  the  area  the  conversion  of  the  shales  to  jaspers  where 
they  are  in  contact  with  the  diabase  is  evidence  that  the  fuma- 
rolic  action  accompanying  the  injection  of  diabasic  magma 
was  manifested  by  aqueous  and  probably  siliceous  emanations 
and  was  fairly  intense.  It  seems  possible  that  the  operation 
of  the  fumarolic  action  upon  the  elements  already  present  in 
the  magnesian  limestones  might  have  been  sufficient  to  convert 
the  more  siliceous  portions  into  serpentine.  The  occurrence 
of  the  asbestos  in  veins  that  cut  both  the  nodules  of  serpentine 
and  the  limestones  is  evidence  that  the  formation  of  the  cross¬ 
fiber  asbestos  was  itself  a  somewhat  later  phenomenon. 

Age  and  Correlation. 

The  Grand  Canyon  series  is  referred  to  the  Algonkian  in 
the  usage  of  the  United  States  Geological  Survey  and  is  ten¬ 
tatively  correlated  with  the  Keweenawan  series  of  the  Lake 
Superior  region  and  with  the  Llano  series  of  Texas,  following 
the  conclusions  established  by  Walcott  (J,  p.  518).  In  the 
Shinumo  area  the  profound  nature  of  the  unconformity  which 
separates  these  strata  from  the  basal  Tonto  sandstone  is  even 
more  striking  than  in  the  area  described  by  Walcott,  while 
the  certainty  of  their  stratigraphic  position  is  in  its  clearness 
and  spectacular  character  probably  unparalleled  in  the  world. 

Geologic  History. 

Outline. 

The  following  sequence  of  events  may  be  distinguished  : 

1.  Deposition  of  the  Vishnu  sediments,  source  unknown. 

2.  Regional  metamorphism. 

a.  Subsidence  and  profound  burial. 

b.  Orographic  movement  of  folding  and  compression 

resulting  in  recrystallization  and  schistosity,  accom¬ 
panied  or  preceded  by  injections  of  pegmatite. 

c.  Elevation,  accompanying  or  succeeding  the  orographic 

movement. 

3.  Batholithic  invasion  of  quartz  diorite. 

4.  Later  pegmatitic  injections. 

5.  Long  cycle  of  erosion  carried  through  to  a  featureless  pene¬ 

plain  of  no  relief,  which  truncates  the  verv  roots  of  the 

Vishnu  structure. 


The  Shinumo  Area. 


523 


6.  Sudden  incoming  of  the  shallow  Unkar  sea. 

7.  Deposition  of  the  Unkar  sediments  and  succeeding  strata  of 

the  Grand  Canyon  series  in  shallow  water  (or  upon  land  ?) 

8.  Intrusion  of  diabase  sills. 

9.  Orographic  movement  of  block  faulting  and  tilting  accom¬ 

panied  or  succeeded  by  elevation. 

10.  Long  cycle  of  erosion  carried  through  to  a  peneplained  sur¬ 

face  of  small  relief. 

11.  Incoming  of  the  Tonto  sea,  succeeded  by  the  deposition  of 

the  Paleozoic  strata  of  the  wall  of  the  Grand  Canvon. 

«/ 

The  earliest  event  which  is  decipherable  in  the  geologic 
history  of  the  Shinumo  area  is  found  in  the  blurred  and 
mangled  record  of  the  Vishnu  schists.  Far  back  in  the  dark 
ages  of  geologic  time  a  thick  series  of  more  or  less  arkose 
kands  and  muds  was  accumulating  upon  a  subsiding  floor. 
So  much  may  be  reasonably  inferred  from  the  mineralogical 
character  of  the  quartz  schists  of  the  mica  and  hornblende 
type.  So  dim  and  vague  is  the  record  that  the  base  of  this 
series,  the  floor  upon  which  it  was  laid  down,  the  thickness, 
and  the  location  of  the  land  mass  from  which  it  was  derived 
must,  perhaps,  remain  forever  unknown.  Following  the  long 
accumulation  and  burial  of  these  sediments  came  an  orographic 
movement  which  wrote  across  the  older  manuscript  in  a  newer 
and  bolder  hand,  blurring  the  ancient  writing  with  the  stamp 
of  deep-seated  regional  metamorphism,  and  imparting  to  the 
manuscript  the  aspect  of  a  palimpsest.  The  regional  meta¬ 
morphism  is  conceived  to  have  been  brought  about  by  deep 
burial  of  the  sediments,  followed  by  folding  and  compression, 
which  engraved  upon  them  the  characters  of  recrystallization 
and  schistosity  in  slow  process  of  time,  and  accompanied  by 
their  elevation  into  lofty  mountains.  Somewhat  later  the  cores 
of  these  mountains  were  intruded  by  great  masses  of  igneous 
rock,  here  in  the  form  of  quartz  diorite,  followed  by  peg- 
matitic  injections.  Perhaps  while  this  orographic  movement 
was  still  in  progress  the  forces  of  erosion  were  already  at  work. 
Then  followed  a  tremendous  cycle  of  erosion  carried  through 
to  the  very  end,  planing  away  the  ancient  mountains  to  the 
basal  roots,  and  reducing  hard  and  soft  rocks  alike  to  an  utterly 
flat  and  monotonous  level.  Such  was  the  completion  of  the 
cycle  of  this  vast  unknown  and  unnamed  seon  of  time. 

The  next  event  is  the  beginning  of  another  great  cycle  of 
sedimentation  resulting  in  the  deposition  of  the  Grand  Canyon 
series  of  Unkar  and  Cliuar  strata,  ushered  in  by  the  sudden 
invasion  of  a  shallow  sea  which  swept  over  the  featureless 
desert  surface  of  the  Vishnu  plain,  depositing  the  basal  conglom¬ 
erate  of  the  Unkar.  The  clue  to  the  inferences  as  to  the 
character  of  this  incoming  sea,  of  the  rock  mantle  which  it 


524  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

found  covering  the  plain,  and  of  the  climate  of  the  time  is 
preserved  for  us  in  the  basal  conglomerate  described  in  a 
previous  part  of  this  article.  It  was  there  noted  that  the 
weathering  below  the  surface  of  the  peneplain  was  slight,  and 
the  product  of  physical  disintegration  rather  than  of  chemical 
weathering ;  that  the  matrix  of  the  conglomerate  is  red  in  color 
and  arkose  in  nature;  that  the  pebbles  are  angular  and  show 
no  evidence  of  transportation  and  sorting.  We  may  therefore 
conclude  that  the  conglomerate  represents  the  soil  in  place 
which  covered  the  Vishnu  plain.  All  the  evidence  points 
to  an  arid  climate, — the  lack  of  chemical  weathering,  the 
freshness  of  the  arkose  feldspar,  and  the  red  color  of  the  matrix, 
all  indicating  a  lack  of  vegetation  and  abundant  moisture  which 
could  decompose  the  soil  and  reduce  the  iron  which  imparts 
the  red  color.  It  is  therefore  not  unlikely  that  the  Yishnu 
plain  was  a  vast  desert  at  the  incoming  of  the  sea.  The  abso¬ 
lute  lack  of  transportation,  sorting,  and  rounding  of  pebbles 
indicates  that  the  incoming  sea  had  little  chance  to  rework 
the  soil  mantle  by  its  waves.  It  seems  impossible  to  account 
for  this  phenomenon  except  by  a  sudden  invasion  of  the  sea 
across  the  Yishnu  desert.  If  we  interpret  the  past  in  the  light 
of  the  present  our  only  guide  is  to  seek  to  picture  some  present 
condition  on  the  earth  which  parallels  that  preserved  in  the 
geological  record.  A  possible  clue  may  lie  in  the  conditions 
about  the  Caspian  Sea  to-day.  There  is  in  that  region  a  desert 
about  the  shores  of  the  Caspian  which  lies  below  the  present 
level  of  the  Black  Sea ;  a  sudden  rise  in  the  level  of  the  ocean 
might  cause  the  latter  to  overflow  the  low  barrier  which  sepa¬ 
rates  it  from  the  Caspian,  and  in  this  way  a  sudden  inundation 
of  the  desert  would  be  accomplished.  It  is  now  thought  that 
wind  erosion  may  carry  the  surface  of  an  old  desert  to  a  level 
below  that  of  the  sea  if  the  ctycle  is  continued  to  extreme 
maturity.  Such  forces  might  have  been  active  in  the  last 
stages  of  the  Yishnu  peneplain. 

The  deposition  of  the  alternating  limestones  and  shales  of 
the  second  member  of  the  Unkar  seems  to  have  taken  place  in 
a  permanent  water  body  into  which  mud  was  frequently 
washed.  It  may  be  noticed  from  the  section  of  this  member 
that  the  alternations  are  almost  innumerable.  The  exact  cause 
of  this  is  speculative.  Possibly  the  alternations  are  due  to 
climatic  oscillation  :  a  movement  from  an  arid  to  a  semi-arid 
climate  would  load  the  rivers  with  sediment,  while  arid 
intervals  would  retard  their  flow,  if  not  dry  them  up  entirely, 
resulting  in  a  temporary  clarifying  of  the  sea  and  a  deposition 
of  limestone.  Whether  the  limestones  are  the  result  of  organic 
agencies  or  of  purely  chemical  precipitation  is  also  a  matter  of 
speculation,  since  in  regard  to  decisive  evidences  of  life  the 


I 


The  Shinumo  Area. 


525 


geological  record  is  silent.  It  is  believed  that  the  water 
body  was  for  the  most  part  shallow,  since  sun-cracks  appear  in 
the  shales  in  the  upper  part  of  the  member  below  the  highest 
limestone  strata.  What  the  polygonal  cracks  and  the  small 
cubic  depressions  in  the  banded  cherty  layers  of  the  basal  lime¬ 
stones  mean  the  writer  does  not  know.  The  cracks  are 
suggestive  of  sun-cracks  and  the  depressions  stroneflv  resemble 
salt  hoppers. 

The  most  striking  feature  of  the  third  member  is  the  great 
abundance  of  sun-cracks  throughout  the  shaly  strata  and  of 
ripple-marks  and  of  cross-bedding  in  the  sandstones.  The 
researches  of  Professor  Barrell  have  made  it  clear  that  exten¬ 
sive  sun-cracking  is  a  feature  which  has  a  maximum  chance  of 
preserval  on  broad  flood  plains  or  deltas  in  an  arid  climate. 
(Barrell,  a,  pp.  524-568.)  In  the  opinion  of  the  writer  of  the 
present  paper,  the  extreme  abundance  of  these  cracks  in  this 
member  of  the  Unkar  is  hard  to  account  for  except  by  postu¬ 
lating  wflde  delta  flats  or  flood  plains.  Furthermore,  the  bright 
red  color  of  the  shaly  strata  in  connection  with  the  mud  cracks 
seems  to  bespeak  an  arid  climate  with  little  or  no  vegetation 
to  reduce  the  iron.  It  is  at  least  certain  that  all  this  part  of 
the  Unkar  was  deposited  in  very  shallow  water  which  often 
evaporated  entirely,  leaving  broad  mud  flats  exposed  to  a 
hot  sun.  In  the  upper  part  of  the  member  is  the  series  of 
alternating  shales  and  sandstones  already  described  in  detail : 
as  may  be  seen  from  the  sections,  the  alternations  are  fre¬ 
quently  as  regular  as  clockwork.  The  sandstone  layers  are 
composed  of  fine,  cleanly  sorted  and  rounded  quartz  grains, 
ripple-marked  and  cross-bedded,  while  the  shales  are  a  fine  red 
mud.  It  is  thought  that  the  clean  character  of  the  sandstone 
layers  of  this  alternating  series  is  a  mark  of  climatic  oscilla¬ 
tion  :  a  climatic  movement  towards  a  wetter  climate,  if  increas¬ 
ing  the  ratio  of  run-off  to  erosion,  causes  the  rivers  to  flow  on 
a  lower  grade  and  sweep  seaward  the  piedmont  deposits  of 
sand  and  gravel ;  as  the  clay  wras  largely  sorted  from  those 
deposits  when  they  were  first  laid  down,  their  redistribution 
accompanied  by  a  secondary  sorting  on  a  delta  surface  or  sea- 
bottom  would  be  marked  by  extreme  cleanness. 

Great  thickness,  clear  sorting,  and  extreme  fineness  and 
roundness  of  grain  are  the  characters  which  distinguish  the 
fourth  member,  which  is  composed  entirely  of  sandstones  and 
quartzites.  All  strata  are  characterized  by  cross-bedding  and 
ripple-marks,  bespeaking  shallow  water.  The  origin  of  these 
great  thicknesses  of  sandstone  is  a  puzzle.  The  clean  sorting 
seems  to  indicate  long  transportation  :  it  is  not  impossible  that 
the  rivers  carrying  this  material  might  have  flowed  through  a 
great  desert  of  dune  sands,  picking  up  and  carrying  material 


526  Noble — Geology  of  the  Grand  Canyon ,  Arizona, 

such  as  is  deposited  in  the  Indus  delta  to-day  from  a  similar 
source.  Occasional  lenses  of  fine  conglomerate  within  some 
of  the  strata  suggest  scoured  and  filled  stream  channels. 

The  upper  member  of  the  Unkar  again  bears  all  the  marks 
of  shallow  water  origin  :  mud-cracks,  ripple-marks,  and  cross¬ 
bedding  characterize  the  entire  thickness.  The  addition  of 
micaceous  material  and  of  some  feldspar  gives  a  slightly  arkose 
character  to  the  rock ;  possibly  a  crustal  movement  rejuve¬ 
nating  the  land  mass  supplying  the  sediments  was  responsible 
for  the  change  in  character.  Here  again  are  marks  of  aridity, 
seen  in  the  vermilion  color  and  the  vast  development  of 
mud-cracks. 

All  subsequent  Unkar  and  Chuar  deposits  have  been  removed 
by  the  truncation  of  the  pre-Cambrian  structure  by  the  plane 
of  the  base-leveled  surface  of  erosion  beneath  the  Tonto 
Sandstone. 

In  summary  it  may  be  said  that  the  evidence  obtainable 
from  the  lithologic  record  of  the  Unkar  sediments  in  the 
Shinumo  Area  points  probably  to  an  arid  climate,  and  almost 
surely  to  deposition  in  shallow  waters ;  first  in  a  permanent 
body  of  water  and  later  in  deltas  or  on  flood  plains.  Which 
of  the  latter  conditions  prevailed  the  writer  is  not  competent  to 
say. 

The  predominance  of  clastic  sediments  instead  of  limestones 
in  the  basal  portion  of  Walcott’s  section,  30  miles  east,  sug¬ 
gests  that  that  area  was  nearer  to  the  shore  line  of  the  early 
shallow  sea.  The  close  correspondence  of  the  stratigraphic 
succession  and  lithology  in  all  higher  members  in  the  two 
areas  suggests  uniform  conditions  over  at  least  that  distance. 

The  next  event  which  can  be  read  from  the  geologic  record 
is  the  invasion  of  the  Unkar  strata  by  a  thick  sill  of  diabase 
in  the  lower  members,  and  by  four  thin  sills  of  basaltic  rock  in 
the  upper  part  of  the  section. 

Following  this  came  an  orographic  movement  of  block 
faulting  and  tilting,  accompanied  or  succeeded  by  elevation, 
breaking  the  strata  of  the  Grand  Canyon  series  into  great 
crustal  blocks,  and  throwing  them  into  high  ranges  of  moun¬ 
tains  which  in  character  and  aspect  were  probably  not  unlike 
the  faulted  ranges  of  the  Great  Basin  or  the  desert  ranges  of 
Arizona. 

Then  began  a  second  vast  interval  of  erosion,  gnawing 
slowly  but  surely  into  these  faulted  mountains,  reducing  them 
in  slow  process  of  time  through  stages  of  youth,  maturity,  and 
old  age,  and  finally  planing  away  all  except  the  very  hardest 
strata  of  their  cores  to  form  the  broad  monotonous  expanse  of 
a  base-leveled  surface.  The  monotony  of  this  surface  was 
broken  only  by  an  occasional  monadnock  of  hard  Unkar  quartz- 


The  Shinumo  Area. 


527 


ite,  resisting  the  forces  of  erosion  in  that  ancient  plain  by 
virtue  of  the  terrific  hardness  which  causes  the  same  strata 
to-day  to  wall  in  the  deep  box-canyon  of  the  Shinumo.  These 
monadnocks  of  the  Cambrian  plain  may  be  compared  with  the 
Baraboo  ridges  of  Huronian  quartzite  which  by  virtue  of  their 
homogeneity  and  hardness  still  stand  as  prominences  which 
have  weathered  repeated  cycles  of  erosion.  It  is  probable 
that  this  cycle  of  erosion  was  not  finally  completed  until  well 
along  in  lower  Cambrian  time. 

The  next  event  is  the  incoming  of  the  Tonto  sea.  Although 
this  chapter  belongs  to  the  Paleozoic  history  recorded  in  the 
horizontal  strata  of  the  walls  of  the  Grand  Canyon,  that  part 
which  is  involved  with  the  distribution  of  the  Unkar  strata 
upon  the  pre-Tonto  surface  may  properly  be  anticipated  here. 

It  is  not  unlikely  that  this  invading  sea  transgressed  a  sur¬ 
face  which  strongly  resembled  the  present  surface  of  the 
great  Lauren tian  peneplain  of  Canada  with  its  broad  areas  of 
crystalline  rocks  in  which  are  inset  occasional  blocks  of  Paleo¬ 
zoic  strata,  and  above  which  stand  occasional  monadnocks  of 
quartzite.  The  story  of  the  invading  sea  is  written  in  the 
Tonto  Sandstone,  locked  up  in  which  is  a  record  of  marine 
transgression  which  on  account  of  the  vertical  sections  and 
absence  of  soil  is  clear  beyond  belief.  When  the  Tonto  sea 
came  in  over  the  surface  of  the  ancient  peneplain  the  monad¬ 
nocks  stood  out  as  islands  which  were  gradually  overwhelmed 
and  buried  in  the  sands  of  the  deepening  sea.  The  long  ridge 
of  the  quartzites  of  the  middle  Unkar  in  the  Shinumo  area  has 
already  been  described.  It  stood  out  as  a  long  narrow  rocky 
island  whose  longer  axis  extended  for  an  unknown  distance 
northwestward  along  the  strike  of  the  strata.  The  long  south¬ 
west  face  of  this  island  was  undercut  by  the  marine  planation 
to  form  a  steep  cliff.  Every  detail  of  the  face  of  this  old  sea 
cliff  is  preserved  in  the  Tonto  sandstone  in  the  cross-sections 
cut  by  the  present  canyons  across  its  face :  at  its  base,  huge 
angular  blocks  of  Unkar  quartzite  as  large  as  houses  are  pre¬ 
served  in  the  Tonto  sandstone  in  the  exact  position  where  they 
fell  and  lodged  ;  farther  out  from  the  base  are  masses  of  large 
bowlders,  worn  and  rounded  by  the  pounding  of  the  waves ; 
these  become  smaller  and  smaller  and  finally  run  out  into 
lenses  of  fine  pebbly  conglomerate,  representing  the  shingle  of 
the  ancient  beach,  dragged  out  by  the  undertow.  Uo  more 
striking  example  of  a  fossil  sea  cliff  can  be  imagined. 

By  far  the  most  impressive  feature  of  this  wonderful  coun¬ 
try  is  to  traveller  and  geologist  alike  the  mile-deep  pathway 
of  the  Colorado  River  of  the  West  across  the  great  plateaus. 
The  stupendous  and  glaring  record  of  erosion  revealed  to  us 


528  Noble — Geology  of  the  Grand  Canyon ,  Arizona. 

in  the  cutting  of  this  mighty  gorge  has  almost  blinded  ns  to 
the  realization  of  the  immensity  of  the  vastly  greater  record 
revealed  in  its  walls.  But  the  story  told  by  the  two  intersect¬ 
ing  unconformities  in  the  bottom  of  the  gorge, — two  ancient 
cycles  of  sedimentation,  uplift,  and  erosion  carried  to  a  finish, 
separated  by  hopelessly  lost  intervals  of  time,  resulting  twice 
in  the  planing  down  of  lofty  mountain  ranges  to  the  very  core, 
written  vaguely  at  first  on  a  blurred  and  time-worn  record,  and 
later  in  an  increasingly  clearer  and  bolder  hand, — the  slow 
accumulation  of  the  strata  of  the  Canyon  wall  on  the  floor  of 
the  Paleozoic  sea,  the  subsequently  erased  record  of  the  accu¬ 
mulation  of  vast  thicknesses  of  Mesozoic  and  Eocene  strata, 
separated  in  turn  by  great  intervals  of  erosion,  and  even  the 
“  great  denudation  ”  which  has  stripped  these  later  strata  back 
fifty  miles  to  the  terraces  of  Utah — represents  a  lapse  of  time 
compared  with  which  the  cutting  of  the  Grand  Canyon  is  but 
the  passing  of  an  afternoon  ;  for  in  the  light  of  present  knowl¬ 
edge  it  is  safe  to  say  that  the  Grand  Canyon  was  entirely  cut 
since  the  time  when,  according  to  the  fossil  record,  the  remains 
of  man  first  appear  on  earth. 

Bibliography. 

Barrell,  J.  a.  Journal  of  Geology,  vol.  xiv,  1906. 

D filer,  J.  S.  a.  The  Production  of  Asbestos  in  1907.  Mineral 
Resources  of  the  U.  S.  for  1907.  U.  S.  Geol.  Surv. 

b.  The  Production  of  Asbestos  in  1908.  Mineral 
Resources  of  the  U.  S.  for  1908.  U.  S.  Geol.  Surv. 
Dutton,  C.  E.  a.  Tertiary  History  of  the  Grand  Canyon  Dis¬ 
trict,  with  Atlas.  Monogr.  II,  U.  S.  Geol.  Surv.  1882. 
Ransome,  F.  L.  a.  Pre-Cambrian  Sediments  and  Faults  in  the 
Grand  Canyon  of  the  Colorado.  Science,  vol.  xxvii,  No. 
695.  April  24th,  1908. 

b.  Geology  of  the  Globe  Copper  District,  Ariz.  Prof. 
Paper,  No.  12,  U.  S.  Geol.  Surv. 

Walcott,  C.  D.  a.  Study  of  a  Line  of  Displacement  in  the 
Grand  Canyon  of  the  Colorado  in  Northern  Arizona.  (Read 
Aug.  29,  1889.)  Bull.  Geol.  Soc.  America,  I,  1890  (Feb.), 
pp.  49-64. 

b.  Pre-Cambrian  Igneous  Rocks  of  the  Unkar  Terrane, 
Grand  Canyon  of  the  Colorado,  Arizona,  by  Charles 
D.  Walcott;  with  notes  on  the  Petrographic  Char¬ 
acter  of  the  Lavas,  by  Joseph  P.  Iddings.  14th 
Ann.  Rept.  U.  S.  Geol.  Surv.  for  1892-1893,  Pt.  2, 
1894,  pp.  497-519  (Walcott)  and  520-524  (Iddings), 
pis.  lx-lxv. 

Auburn,  N.  Y.,  January,  1910. 


Am.  Jour.  Sei.,  XXIX,  1910. 


Plate  V. 


36°  30' 


36°  15' 


112°  30' 


112°  15 


Map  showing  the  Shinumo  area  (within  double  ruled  lines)  and  distribution  of  Grand  Canvon  and  Vi«h™  ^  ,,  , 

graphic  sheets,  U.  S.  G.  S.  Contour  interval,  250  ft.  Scale  about  5£  miles  per  inch.  ^  eries.  From  the  Kaibab  and  Echo  Cliffs  topo- 

~T  *  Grand  Canyon  Series:  Cliuar  Group,  cross  hatched  Unkar  Groum  cnlirl  V.iar.u  •  t  ^  n  , 

area  ;  III,  Bright  Angel  area  ;  IV,  Hindu  area  ;  V,  Shinumo  area  ;  VI,  Tapeats  area.  Vishnu  Series,  x  x  k  ’  TjPe  ^  described  b>T  Walcott ;  II,  Ottoman 


, 


\ 


